Scoffed at from its start for setting itself up as a rival to the stodgy, old CBS, NBC and ABC, Fox celebrates 25 years on the air this Sunday with a two-hour special featuring cast reunions, re-runs, highlights and some major trumpet blowing.

But even as it reaches that milestone, the TV landscape Fox helped change with "The Tracey Ullman Show," "Married With Children," "Ally McBeal" and "24," is further transforming and giving the News Corp.-owned network a new set of rivals on still-expanding cable TV and the internet.

"When this network started, no-one thought a fourth U.S. network was even possible. I think we were laughed at in the beginning," Mike Darnell, Fox's head of alternative programming, told Reuters. "This is a coming of age. This is the first anniversary where we feel we've been on long enough to hold a celebration."

Fox has plenty of history to cheer. After a wobbly start, shows such as "The Simpsons" and later "Beverly Hills, 90210" became youthful, pop culture hits and lured large audiences. Currently, it remains the most-watched network in the 18-49 age group coveted by advertisers, where it has been for seven consecutive years. And it is on track to win its eighth season with those viewers behind highly-rated "American Idol."

"Idol" - the singing contest that executives took a chance on as a summer filler in 2002 after its rivals said no - has been the most-watched show on U.S. TV for eight years. And while many industry watchers believe it has now morphed into a comfortable, middle-aged program with a declining audience, it was the kind of risk that epitomized Fox in its early days.

"Fox - the fox in the hen house, the sly one - was an extremely appropriate name for the network," said Tim Brooks a former TV executive and co-author of the "Complete Directory to Primetime Network and Cable TV shows."

GOOD STRATEGY; GOOD SHOWS

The network derived its name from 20th Century Fox, the movie studio that is part of Rupert Murdoch's News Corp media empire, and initially earned a financial boost by programming one hour less in primetime than its rivals and by broadcasting mostly in urban areas versus reaching deep into rural America - both moves that helped reduce costs.

On U.S. airwaves, the fledgling channel found its niche with young people who were bored with the cloned sitcoms and dramas offered by the major broadcasters. Along came shows like "In Living Color," "X Files" and others that pushed the boundaries of mainstream TV.

Fox also explored genres not regularly on primetime. "Animation had been the stuff of Saturday mornings," said Robert Thompson, pop culture professor at Syracuse University. "Fox put on 'The Simpsons' and completely changed the state of animation for adults."

"The Simpsons", the longest running American primetime scripted show, will be one of the centerpieces of Sunday's 25th anniversary special. The cast of "Married With Children" will reunite, as will the stars of "That '7Os Show" and other celebrities who built their fame with Fox -- David Duchovny, Gillian Anderson, Calista Flockhart and Jason Priestley.

"These were all such important shows for the network and its lovely to get them all back together," said Darnell. "The clips play fantastic and since I have been here a really long time, it is a little emotional for me."

"ALIEN AUTOPSY" ERA

Long before ABC launched game show "Who Wants to be a Millionaire" in 1999 and CBS introduced "Survivor" in 2000, Darnell was putting on his "crazy shows" like "Alien Autopsy," "World's Wildest Police Videos" and "When Good Pets Go Bad."

"In reality TV, people are always accusing you of crossing the line. I cannot tell you how many articles have been written about the end of western civilization, about a lot of the shows I did in the past," said Darnell.

"But it became mainstream, the more people did it. When the reality genre was in its infancy, it was easier to come up with things that would shock or be controversial. Now the genre has matured and the more shocking material has been taken over by cable TV," he said.

Thompson said that despite angry protests from parents groups and raised eyebrows over the years, "Fox has made some really important contributions to the history of television. I think some of that stuff was really exciting.

"But Fox is no longer that guerrilla, you-never-know-what-you-are-going-to-see network," he said. "A network can only be avant-garde for so long."

Darnell has a different view. He points to shows like musical comedy "Glee", which became a pop phenomenon three years ago and paved the way for "Smash" on NBC, sci-fi drama "Touch" and quirky comedy "Raising Hope" as Fox's new touchstones.

"I think we started out and still remain the maverick of the TV networks ... It doesn't have to be controversial or edgy but it has to be new. What Fox has really become known for is trying new things, achieving some success and getting copied by other networks," he said.

Still, the network now faces challenges never dreamed about in 1987. Digital video recorders and Internet services such as Hulu and Netflix give audiences an ever-increasing number of ways to consume their favorite programs and bypass the lifeblood of network TV - advertisers of mass-market, consumer products.

And in an age of fragmented cable TV where channels narrowcast to small audiences of animal lovers, foodies and fans of Oprah Winfrey, finding the kind of headline-grabbing shows that helped launch Fox is getting harder, media watchers say.

"There is something in the DNA of Fox that wants to be what it was when it was born, which is the rude kid in the playground that is doing outrageous things even though they know that, now they are 25 years old, they have to act adult," said Brooks.

(Reporting By Jill Serjeant; Editing by Bob Tourtellotte)

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