Just two years ago, the Fox network proclaimed a bold new concept in summer television programming: an original show. Staggering into the endless Sahara of repeats, the network debuted a big-budget action series called "Roar" in mid-July. The show, said Fox Entertainment President Peter Roth, was "a sign of the future for all the networks. Network television can no longer shut down for the summer and drive viewers to cable."

Good idea, bad execution. "Roar" merely whimpered, and when viewers failed to show up in sufficient hordes, Fox dropped the show and, with it, the summer originals strategy. A few months later, it also dropped Peter Roth.

Message: Reruns rule.

When it comes to summer, the leading broadcast networks figure old and worn beats fresh and new. An occasional new series will materialize on the major networks this summer--ABC has a new game show, "Who Wants to Be a Millionaire?," scheduled for mid-August--but viewers will mostly eat repeats from now until the new season starts in mid-September.

Viewers may be turned off to all this rehashing, but the networks can't live without it. Privately, rival broadcast executives cheered the demise of "Roar." Had Fox been able to establish a slate of original programs, it might have upset one of the great, and perhaps last, lucrative outposts in network TV.

The fact is, summer reruns are the most profitable kind of programming carried by ABC, NBC, CBS and Fox. Because the cost of original programming is so high (NBC pays an extraordinary $13 million for each new episode of "ER"), broadcasters don't recoup their investment until a show goes into its second or third airing.

But here's the paradox of the repeat: Reruns may be wounding network TV even as they're delivering the profits.

Like a progressive disease, reruns eat away at the networks' loyal audiences; they "add to viewer confusion, interrupt viewing patterns, encourage channel switching, and benefit competitors" by turning viewers into nomadic channel grazers, says a new ad-industry study, which details the amount of reruns and their toll on network audiences.

Of course, networks have always aired reruns, primarily because these shows are cost-efficient and because producers can't crank out shows 52 weeks a year.

But reruns aren't what they used to be. Major changes in advertising, audience composition, network competition and the Nielsen ratings have altered the picture.

Mainly, the changes add up to something you'd probably already suspected: There are more repeats, more often, on network TV.

In the 1950s the networks purchased TV series on the basis of 39 weeks of originals and 13 weeks of repeats. In the 1960s the standard ratio was reduced to 26/26. By 1970, according to the Encyclopedia of Television, the first-run order fell to about 22 episodes. That's where it stands now.

In the 1970s, however, the networks produced more specials, more short-term series and other one-time original programs, which limited the number of times they had to resort to repeats. Back then, as well, the television "season"--the period advertisers and networks considered as prime-viewing months--ended in April.

Two things have happened since then: The amount of original programming has been cut back to save on rising production costs, and the "season" itself has shifted.

Nowadays the TV season runs through the end of the May "sweeps," when ratings determine what stations will be able to charge advertisers in the succeeding months. In recent years network affiliates around the country have demanded original programming in May to hype their sweeps period and to compete against cable networks.

This one-month extension has forced the networks to hold on to original episodes longer. With reductions in the number of replacement series and specials, there's little but reruns to plug the holes leading up to May.

After November, "you're playing a cat-and-mouse game, peppering in the originals with the repeats," says Preston Beckman, who heads program planning and scheduling for NBC. "In January, March and April, I've only got two or three originals [of a series] to play with."

Indeed, 41 percent of the shows carried by the Big Four networks in prime time last December were reruns, according to TN Media, an ad agency that analyzed reruns in a study last month. Between 1982 and the current season, reruns on the networks shot up dramatically in every month except April (when the number was slightly down) and in May, according to the study.

This doesn't include still another kind of rerun--syndicated repeats of popular network shows. These kinds of reruns air on local stations after they've been broadcast two or three times nationally, often just weeks after they've finished airing on the network (veteran producer Sherwood Schwartz claims that "Gilligan's Island," a show he created in the early 1960s, is the all-time rerun champion, with each episode having aired thousands of times in the past three decades).

With viewing choices exploding, network executives figure viewers probably haven't seen all of a show's episodes in their original run. NBC tried to institutionalize this approach two summers ago, promoting repeats of sitcoms like "Mad About You" and "Frasier" with the slogan "If you haven't seen it, it's new to you."

But the networks also recognize that this strategy is slowly driving away loyal viewers. "In the old [TV] environment, you could take a favorite series off for six weeks to try something new," says Kelly Kahl, CBS's chief program planner. "There were only a couple of places viewers could go. Now if a show's gone for six weeks, fewer people come back. We're setting viewers free to go looking" for other options.

This summer the effect of the repeat doldrums has been dramatic. The four broadcast networks suffered their lowest collective rating ever in the week leading up to the Fourth of July weekend. For NBC the most alarming trend has been the steady drain of viewers from repeats of "ER," the most expensive program on TV.

Reruns of that show on NBC have fallen 33 percent so far this summer compared with last summer. It has even been beaten in overall household ratings three of the past four weeks by CBS's "48 Hours," which, like most newsmagazine shows, continues to carry fresh material during the summer.

Of course, this is an "ER" that no longer can count on the hefty lead-in audiences of "Seinfeld"; that no longer has George Clooney in its cast; and that has been hurt by smaller-than-usual audiences for the post-Michael Jordan NBA playoffs, which NBC counts on each year as a platform to promote its summer shows.

But reruns of "ER" may also be hurt by . . . reruns of "ER." Old episodes of the medical drama air nightly on cable's TNT, which some industry watchers say may be creating "ER" overload. Structurally, too, "ER" could be at a deficit; serial dramas, NBC's Beckman acknowledges, historically don't repeat as well as "self-contained" dramas (think "Law and Order") or sitcoms.

The networks' summertime blahs have, in turn, become cable's party season. Taking advantage of the broadcasters' slumber, cable channels regularly launch their new series in June and July. Among many others, HBO last week began airing new episodes of its prison series, "Oz"; MTV is launching a new half-hour series called "Undressed," which focuses on discussions that occur while couples are getting ready for bed; and tonight the USA Network premieres "GvsE," a kind of hybrid police and science fiction series.

Cable has plenty of repeats, too (episodes of "The Sopranos" are in heavy rotation on HBO), but cable has become a welcoming port for disaffected broadcast viewers. By seeding a few original series throughout prime-time hours, cable networks have cushioned the effects of the inevitable summer tune-out. On average, ratings for the broadcast networks fall 25 percent during the summer--and rise 8 percent for basic cable networks.

So what's a broadcast television network to do?

In some ways the networks' programming decisions over the past decade reflect their efforts to deal with the new rerun realities.

ABC's top program planner and scheduler, Jeff Bader, points to the rise of game shows, "reality" shows and newsmagazines such as NBC's "Dateline" and ABC's "20/20." All of these genres are relatively inexpensive to produce, he said, and production can take place almost year-round. News-driven shows, which mix in new and old material during the summer, are among the highest rated; last summer, they averaged an 8.1 rating--a full 37 percent better than repeats of continuing entertainment series.

"In a perfect world, we'd have more original episodes of the shows people want to see," said Beckman. Given that that's economically unrealistic, "we've tried to build a strong news presence in prime time."

Concludes TN Media's Steve Sternberg in his study: "Only by consistently airing more originals of established series during the summer (or airing newsmagazines every night) will the networks be able to improve their summer performance. . . . If they can figure out how to profitably do that, [advertisers] will applaud them."

And so will viewers.

CAPTION: "Gilligan's Island" is believed to be the all-time rerun champion.

CAPTION: Promotional campaigns aside, they're not so new to you: The crew of "Friends," above, and "Frasier's" David Hyde Pierce and Kelsey Grammer.

CAPTION: Keeping it fresh: Eamonn Walker, above left, and Terry Kinney in HBO's "Oz," which just launched its third season; Calista Flockhart and Connie Chung in a "20/20" interview this past spring; and Donatella Versace interviewed by Katie Couric, below, on "Dateline NBC."

CAPTION: Heath Ledger in Fox's ill-fated "Roar," an action-adventure series that debuted--and died prematurely--in the summer of '97.

CAPTION: Play them again: Summer's rehashed prime-time lineup includes, clockwise from top left, NBC's top-rated "ER," Fox's "Melrose Place," NBC's "Mad About You" and ABC's "NYPD Blue."