During Part 2 of TV reporter Mark Feldstein's theatrical investigative series about cash bribes allegedly paid to District taxicab inspectors, which has been airing all week on Channel 9's nightly newscasts, Mayor Marion Barry turns in a stunning performance.

It is the sort of moment that the gangly, rail-thin Feldstein relishes. Barry is standing before the podium at his monthly press conference, staring into the television lights that surround him. Feldstein stands to ask a question about the alleged payoffs to D.C. officials he has depicted on television the night before.

"This administration has basically been scandal-free," Barry says, and he accuses Feldstein of breaking the law by failing to report any corruption he has found to the police. The mayor is sweating now, perhaps as much from the warm lights as from the heated questioning. And just as he says the word "scandal," he pulls a handkerchief from his pocket and begins to wipe his brow. He mops his nose, then his mouth, his neck and his eyes.

That night, the mayor's perspiration is beamed to all of Washington.

"I was amazed at his reaction," a gleeful Feldstein said yesterday in his cramped office at WDVM-TV, a few hours before the third part of his "Greasing the Wheels" series aired. "He acted in a very defensive manner."

Of course, Mayor Barry's sweaty brow says absolutely nothing about his culpability in any alleged scandal -- but it does provide the kind of visual drama that is essential to Feldstein's work. And juxtaposed against the pictures Feldstein has broadcast of men with nicknames like "Lefty" and "Cocky" allegedly taking cash to pass faulty cabs through inspection, it adds greatly to the "impact" Feldstein says he strives for in his investigative reports.

"There are some things that just don't translate in print the way they translate on television," he says.

Like the way Feldstein burst into the office of unsuspecting D.C. cab inspectors during last night's report, declaring, "MARK FELDSTEIN, CHANNEL 9," waving his camera around the room like a loaded shotgun. The startled inspectors refused to comment on Feldstein's accusations. One of them finally told Feldstein sternly, on camera, "I advise you to get out of my face."

"I'm not a fan of the so-called ambush interview," Feldstein says. But "when you're talking about people who are allegedly engaging in criminal activity, you don't want to give them a whole lot of warning to rehearse their stories."

Already the winner of two Peabody Awards at 28, Feldstein came to WDVM a year ago from ABC News in New York. He has previously done WDVM investigative series on an unlicensed abortion clinic and a local mental hospital.

Feldstein and his crew spent months staking out the city's inspection sites. "We were in this van with tinted glass just feet away from where the bribes were allegedly going down. Sometimes people would come up and peer in the windows [of the van] and you'd hold your breath and just hope that nobody noticed."

Feldstein's reports have about them the declarative bluntness that is the source of television's impact -- and its limitations. So far, each installment in his series has begun with a picture of a man, his face blackened to protect his identity, holding out a wad of cash to an alleged "middleman" who will use the money to "buy" an inspection sticker.

"THIS IS A CASH PAYOFF," Feldstein intones loudly at the introduction of his first segment. "IT'S DESIGNED TO CORRUPT A SYSTEM THAT'S SUPPOSED TO PROTECT THE PUBLIC . . . "

Such hyperbole is itself designed partly to boost WDVM's ratings during the current "sweeps" rating period, when each of the local stations airs the most compelling and salacious reporting it can muster -- during the last few weeks, Channel 4 has offered reports on AIDS and dangerous household products, while Channel 7 has weighed in with an eight-part series on terrorism worries in Washington.

Still, Feldstein says that he pursues his work mainly because he likes to get the bad guys. "I hate it when people lie," he says. "And we have the time to go find out when people are doing that."