Don't fall in love with "Shasta McNasty."

Yesterday's announcement that UPN parent Viacom would merge with CBS Corp. throws into question the future of the fledgling network and its lineup of male-skewed shows like "Star Trek: Voyager" and the new "WWF Smackdown!" as well as African American-led sitcoms, including "Moesha" and the new "Grown Ups." Not to mention the eagerly anticipated "Shasta McNasty," about a mediocre hip-hop band.

Viacom CEO Sumner Redstone and CBS CEO Mel Karmazin, who would become president and chief operating officer of Viacom if the deal goes through, both said yesterday that they'd like the new combined company, which will be called Viacom, to hang on to UPN.

And UPN was quick to issue a statement that it had received "strong assurances from its parent companies that they are fully committed to the network's long-term operations and prosperity."

But there's that pesky Telecommunications Act of 1996 that may make it illegal for Viacom to own both CBS and UPN. And if the big guys have to choose, they sure as heck aren't going to choose the No. 6-ranked network over the No. 1-ranked one.

For starters, the Telecom Act prohibits a company from owning broadcast TV stations that collectively reach more than 35 percent of the U.S. television audience. If the merger goes through, the UPN stations that are already owned by Viacom and the CBS-owned stations combined would reach nearly 42 percent of the audience.

Then there's the Telecom Act provision that prohibits any of the Big Four networks from buying either UPN or WB--a restriction intended to promote diversity in the broadcast industry. Viacom is a 50 percent owner of UPN, in partnership with a station group called Chris Craft.

Karmazin and Redstone were scheduled to meet today with regulators to seek waivers from both rules.

If they can pull off a waiver of the station cap, they will have accomplished what other networks have tried, but failed, to do for several years. Broadcasters have lobbied long and hard to raise the ceiling to 50 percent. They were foiled by other station group owners, including the Post-Newsweek station group, which lobbied for a 25 percent cap, fearing the networks would become too powerful. Lawmakers compromised with the current 35 percent limit.

On the dual network front, expect the bigwigs to argue that forcing Viacom to put UPN on the block would jeopardize its future, which would be a shame given that it has four shows with African American leads--a lot more than any of the Big Four networks can boast.

This is a hot topic since the NAACP threatened to take legal action against the Big Four over their new white-bread prime-time lineups. Viacom's argument could play well in Washington. UPN's "Moesha" is the No. 10 show among African American viewers; "Malcolm & Eddie" is No. 31. For comparison's sake, NBC's "ER" is No. 17 and "Friends" is No. 106.

The Viacom brass will argue that it's in the African American community's best interest that UPN be owned by a large media conglomerate that can afford to run it as a loss-leader for now. Since in 1998 Viacom lost about $85 million on UPN, for example, there are few buyers with pockets deep enough to take Viacom's share of UPN off its hands, which raises the very real possibility it would be folded, the Viacom execs will say.

In that case, expect Viacom to grab as many UPN stations as possible. That's more stations than you might think because the FCC recently decided to allow the networks to own two stations in some large markets and count that market only once when figuring its total audience reach.

But if Redstone and Karmazin score the waivers and the merger is completed next year, there still may be big changes ahead for UPN. The network has had a bumpy ride since its launch in 1995, largely because of feuding between Paramount and Chris Craft executives and their inability to decide which audience to target. Last season its audience plunged to 2.7 million; at its peak in the '96-97 TV season, it was 4.5 million.

Meanwhile, rival WB has fairly consistently targeted a young female audience. WB averaged 3.6 million viewers in the '96-97 season and by last season had grown to 4.5 million.

UPN's disastrous performance last season cost it millions in May in the upfront market--when advertisers commit to buy time on the networks' fall schedules. UPN took in a mere $140 million, compared with WB's $450 million.

Karmazin is not the kind of guy to tolerate these numbers. But, as Redstone pointed out at yesterday's news conference, UPN's fortunes have improved recently with a schedule that appeals to young male viewers. It has already debuted its new Monday and Thursday lineups, and on Monday it's up 96 percent vs. last fall. On Thursday, with "WWF Smackdown!" it's up about 148 percent.

"Everyone at UPN is energized and rejuvenated by the successful launch of our fall programming, and our parent companies share that enthusiasm," UPN said in its statement.

CAPTION: Viacom's Sumner Redstone, left, and CBS's Mel Karmazin announcing the planned merger of the two media giants yesterday in New York.