IT'S BEEN two whole decades -- the Me and Thee decades, and I'm not sure about Thee -- since Woodstock and such; it's been more than a quarter-century since the Beatles invented the world tour As We Know It. And every summer since, the rock 'n' roll shows have gotten bigger and flashier and louder and more expensive and more costume-conscious and more endorsement-friendly. And suddenly . . . the big summer concerts aren't selling so well. To paraphrase that other Public Enemy, "Is this the end for rocko?"

Well, no; the music industry is alive and well, especially with all those corporate co-sponsors checking in. But it is certainly true that a lot of concert tickets are going begging, not only in Washington but all over. Even the big-star shows are selling more slowly than they used to, forcing promoters to flog harder and longer.

As of a few days ago, for example, there were 3,000 tickets left for Kiss, about a third of what Patriot Center holds; Capital Centre has 5,000 seats left for ex-Zep rep Robert Plant; 7,000 for Fleetwood Mac; and more than 8,000 for Janet Jackson (over three nights). Public Enemy & Co. sold only 1,000 seats on Monday, when tickets went on sale. Plenty of tickets remain for the July 6 Paul McCartney show at RFK, perhaps as many as 20,000 if you include the 4,000 seats bought by Visa for its out-of-a-hat promotion for gold-card customers.

Not that these are inconsiderable sales by any means. Two nights at RFK means 100,000 McCartney tickets. Billy Joel's July 15 show is sold out, and only 700 are available for July 13. Only a couple hundred obstructed seats remain for Madonna's Friday night show at Cap Centre, and Saturday is sold out; on the other hand, promoters had originally discussed three Madonna shows, not two.

It's the second-tier concerts that are struggling. Shows by Tracy Chapman, Rickie Lee Jones, UB40, Whitesnake and Linda Ronstadt are among those mentioned by bookers as ranging from "soft" to "disastrous," depending on the venue. Even the long-rumored David Bowie "Sound & Vision" tour is said to be selling poorly.

Most industry types are blaming the success of big-ticket shows for the troubles mid-level acts are having at the box office. A Billboard magazine story makes a similar point, suggesting that after the megatours come through town, it takes concertgoers a few weeks to save up for another big purchase. Lack of disposable income is certainly a big factor, and it affects more than just rock 'n' roll. Restaurants -- both the expense-account establishments and the more casual happy-hour spots -- have been muttering about a drop-off; one of the fine-arts promoters in town, faced with lagging ballet sales, recently worried that even strong dance supporters are being asked to "support" too many tours.

But there are other factors in play. For one, money attracts money. It's the monster shows -- the ones that in theory would sell out anyway because of the popularity of the performers -- that get the most label support and that garner the most publicity, the most radio and vid-TV hype. (Still, even that market can be saturated: Cher's last show at Cap Centre was half-empty.)

When it comes to entertainers who draw younger audiences, it's the Top 40 successes whom adults (meaning parents) are more often familiar with, and whom they may be more willing to put up the funds for. And now that ticket prices routinely rise to $20 and even $30 apiece, plus the sometimes outrageous prices commanded by souvenir concessionaires, parents are going to be a little picky with allowances.

Even among adult concertgoers, there's a tendency to take a sure bet over the gamble of an alternative or even an up-and-coming artist. As Chesapeake Concerts promoter Michael Jaworek put it, "Maybe when the menu's so big, you decide just to order the prime rib."

But Seth Hurwitz of IMP Productions says the promoters are, in effect, asking for trouble.

"Every year, it's the same thing. At the beginning of the summer there are stories with all the promoters saying how many acts there are and how much money they're going to make on tour and how there's no end to the public's hunger for tickets . . . and then in the summer there's the story about, 'Gee, tickets aren't selling so well' and by the end of the year there'll be the obligatory stuff about how such-and-such was a disappointment.

"They never learn -- there are way, way too many bands touring. There are 200 shows on sale in Washington; the problem is, people do not have an endless amount of money. This should not come as a shock to anybody. Even if every show were great, even if you wanted to go out every night, people have to pick and choose."

IMP, and its flagship 9:30 club, book primarily the "mid-level" and alternative acts Billboard says are suffering. But in the summer, many of those same acts are offered more money at bigger showcases: Erasure, which IMP brought to Bender Arena a few months ago, was offered a lot more to play Merriweather Post Pavilion in July.

"Everybody's in a rush to book these shows," Hurwitz says. "Merriweather is anxious to book shows early so they can announce their subscription packages; Wolf Trap, with its somewhat special {federal} funding, doesn't have to worry about the bottom line the way most promoters do. So the ante goes up."

And the more expensive an act is to book, the more expensive the tickets have to be for the promoter to turn a profit. In other words, it's just another of those vicious inflationary cycles: The acts cost the promoters more; the ticket prices get higher, fewer people show up.

It's also possible that some acts' flacks and handlers may not be serving them as well as they think by pushing them into big circuit tours. There's not a single artist or manager, Hurwitz says, who ever thinks the problems of summer-tour overload will apply to them. "You say, 'Yeah, but you're coming in the night after so-and-so, and the night before this,' and the answer is always, 'Oh, that won't affect us.' "

In the meantime, the bear market in music at least means a good return for investors: Don't be afraid to check on tickets for shows you assumed had long since sold out.