(FORTUNE Magazine) – Puberty is never easy, but it has been particularly awkward for 13-year-old Fox. After years as the brash young upstart of network television, Rupert Murdoch's network has grown powerful enough to make the term "Big Three" obsolete.

But as this past TV season has shown, getting older means dealing with some grown-up problems. Fox, for years a growth story at a time when the rest of network TV was shrinking, has lost 11% of its adult viewers under 50, the ones advertisers pay the most to reach. (Just a year and a half ago, it challenged NBC for supremacy in this demographic.) Its shows--which once appeared so fresh-faced--are losing their charm. Beverly Hills 90210 and Party of Five both ended long runs last month, and even The X-Files and Ally McBeal have begun to fade.

Fox Entertainment Group Chairman Sandy Grushow, the studio executive who now runs the network, says he knows Fox is at a crossroads. The network has to reinvent itself at a particularly difficult time; HBO is now the place producers go to push the limits of TV; the WB and UPN have stolen away some of the young viewers who once turned to Fox by default; and ABC's Who Wants to Be a Millionaire has steamrolled almost everything in its path.

That's why Grushow made a series of bold moves last month intended to right the network. He unveiled a fall lineup with six hours of new programming, the most of any network.

Grushow also tapped Gail Berman, a respected development executive, to run the entertainment division. While president of Regency Television, she developed the WB's Buffy the Vampire Slayer. Berman might want to leave a suitcase near the door, though. The Fox Entertainment president gig is one of the shakiest in Hollywood, with four people cycling through over the past six years. Unlike her predecessor, Doug Herzog, who came to Fox from Comedy Central, Berman has been in business with Fox and Grushow for years. Fox is a 50% partner in Regency Television, which gave the network Malcolm in the Middle, its first bona fide hit in two years. "These are people who are very familiar with me and my style of working," says Berman.

Advertiser response to Fox's moves was lukewarm. The network took in $1.3 billion in "upfront" money, about even with last year. (The networks sell 80% of their advertising inventory the week after setting their fall schedules.) But its share of this pie dipped from 18% to 16%.

Grushow, a TV marketing veteran, knows the challenges ahead of him. The failure rate for new network shows is 80%. He says he and Berman will try to overcome the odds while fashioning a new image for a network that seems to have lost its buzz. "The Fox brand is still the strongest of any broadcast network, but we need to continually feed it," he says. "I'd like to feed it with programs that people will compare to The Simpsons, The X-Files, and Ally McBeal."

His hope for the fall is that two producers who helped build the network can now help rescue it. The schedule will have a new hour from Ally creator David E. Kelley, whose Boston Public centers on a high school faculty. Darren Star, creator of 90210, is back with The Street, featuring young hunks talking about Nasdaq and nookie. Grushow calls it a male version of HBO's Sex and the City and plans a "provocative" campaign to lure female viewers.

Fox will also look for some help from big names in the theatrical film world. The young makers of the wildly successful The Blair Witch Project are providing Fearsum, which will integrate the Internet in its plot lines. Titanic director James Cameron has created a sci-fi series called Dark Angel, starring a radiant newcomer, Jessica Alba. Sounding very much like the Fox of old, Grushow trumpets the show's appeal to women, as well as the "hot, sexy, kick-ass action elements" that will draw in young men. A new Twilight Zone-type horror anthology called Night Visions has first-class casting and production values.

Next season's comedies will be a tougher sell. Schimmel was so widely panned by advertisers that Fox removed it from its fall lineup less than 48 hours after it was introduced. (Fox claims it took off the show because of production problems.) The other, Don't Ask, featuring Roseanne's John Goodman as a gay dad, didn't thrill ad buyers either.

The one thing you won't see much of on the fall schedule is the cheap, sensational reality shows that have become a Fox hallmark. Grushow has been an opponent of the genre since February, after the debacle of Who Wants to Marry a Multi-Millionaire. But he's also a realist--these programs garner boffo ratings. Grushow says a recent quiz show to find the smartest kid in America was just fine. "We have a responsibility to program within the bounds of good taste," he says. "I believe this company has done some things that crossed those lines. It is my goal not to cross those lines."

STEPHEN BATTAGLIO is the senior television correspondent for Inside.com.