Last week NBC suffered the smallest weekly audience numbers in its history. And how is it observing that milestone this week? By airing ads on its own television stations that encourage viewers to switch to a different network.

The ads tout cable's E! Entertainment network, which is now available in about 60 million homes. The 30-second commercials, which begin Tuesday, include testimonials from some of Hollywood's hottest stars.

"E! Channel--they have the latest!" says Tom Hanks.

"Wha'sup E! My favorite channel!" adds Bruce Willis.

E! is running the spots as part of a two-month campaign to plug the expansion of its show "The E! True Hollywood Story" to seven nights a week.

"True Hollywood Story" is considered a big success by E! standards, and at best--on June 6, for example, when it featured the cult TV fave "The Brady Bunch"--the show nabs about 2 million viewers. In broadcast TV terms, those are kinda puny numbers. But it's a bigger haul than scored by UPN on Thursday or Friday nights last week and bigger than last week's audience for WB's reruns of "The Parent 'Hood," "Sister, Sister" or "Smart Guy." But you don't see NBC taking any ads from one of those two networks this week, no sirree.

Why not? Because they're broadcast networks. This makes perfect sense to a broadcast network ad sales guy, but it's a meaningless distinction to viewers.

NBC's policy since 1995 has been to accept advertising from cable networks so long as the ads do not tout a date and time for a specific program.

So while the network spent tens of thousands of dollars' worth of its own on-air time touting its big-budget miniseries "Merlin" for the May sweeps in 1998, the network was also running commercials for HBO's big-budget, 12-part miniseries "From the Earth to the Moon," which one Sunday aired opposite "Merlin."

While that and other cable ads have run on the entire NBC network, the new E! spots are airing only on NBC-owned affiliates. So, in this instance, NBC shooting itself in the foot only in the 28 percent of the country that those stations reach, rather than in the 100 percent of the country that the network as a whole reaches.

NBC isn't alone in its astoundingly out-of-touch thinking that cable doesn't really count. Cable ads litter the broadcast landscape. ABC, for example, accepts cable ads so long as the cable network isn't ad-supported. Which, loosely translated, seems to mean that ABC doesn't want to help other networks steal some of its ad revenue, but doesn't mind if they steal some of ABC's viewers--and will even lend them a hand.

But with broadcast audience numbers shrinking to historic lows, some suits at the broadcast nets are starting to question their companies' thinking--privately.

"The whole notion of who is a competitor and who isn't has all changed . . . in the beginning the thinking was that some of these cable ventures weren't a big concern," said one executive, who wished to remain anonymous.

This would have been the year to spurn cable ads, some execs said. The broadcasters raked in so much money at this May's "upfront market"--when advertisers buy time on the networks' upcoming prime-time schedules--that they could have just said no to cable ads and barely noticed. "It would not have felt like a huge problem," said one critic of the policy.

But that didn't happen. So this season you'll see cable networks and cable programs touted on the broadcast networks. And, as the broadcast nets have continually stressed, there is no more effective place to sway America than on the broadcast networks.

"It's totally dopey," one broadcaster said in disgust.

CAPTION: Tom Hanks and Bruce Willis will tout the E! Channel in ads on NBC.