By RICHARD BERNSTEIN

BEIJING - “Watch out for that sword,” the rehearsal director shouted. “I don’t want anybody’s head getting cut off because you don’t know what you’re doing.”

Lots of weapons were on a stage of the Beijing Opera Academy of China . Teenage future opera stars were armed with lances, spears, swords and daggers as they carried out an elaborately choreographed, intricate, stylized and acrobatic fight scene, all to the clash of cymbals, drums, wooden clappers and a substantial orchestra of Chinese string and woodwind instruments.

Here and there in this ever-moresteel- and-glass metropolis where old neighborhoods disappear from one month to the next, there is a glimpse of what the city used to be like: quiet, treeshaded streets with small storefronts and bicycles, a locust tree leaning over a wall that hides an old courtyard house.

This modest and slightly shabby theater is part of the Beijing Opera Academy, in a neighborhood in the southwest part of the city that has not been entirely torn down and rebuilt yet. And, of course, nothing could more suggest old Beijing than Beijing opera, with its masks, its stylized movements, strangely modern arias, its fantastically intricate scenes of battle, and, probably most important, its audience of connoisseurs who know when to shout a throaty “hao!” - good! - after an especially well-executed movement or song. The worry, though, is that, like the old neighborhoods, Beijing opera could fall victim to China’s rampant commercialism and modernization.

“Objectively speaking, right now there are some difficulties,” said Qiao Cuirong, a senior professor at the National Academy of Chinese Theater Arts, summing up the current state of the opera. “People are interested in money and modernity and Western things, so our own culture has lost something.”

It would be premature to say that Beijing opera has turned into a relic, but clearly it is not what it was in the late 18th to early 20th century, when it was northern China’s most popular theatrical entertainment.

The opera certainly was not helped by the fact that during the turmoil of the 1966-76 Cultural Revolution the form was deemed feudalistic and reactionary. But then again so was just about every other art form, including Western music and modern dance, both of which have since made vigorous recoveries. But the opera faces particular difficulties, aside from the aging and fading away of a knowledgeable audience.

“The more you know about Beijing opera, the more you love it,” said Liu Hua, a former performer and now a teacher at the school. “The problem is that it takes a lot to know it, and fewer and fewer people have the time or the inclination.” Also, Beijing opera is an especially demanding form, both to perform and to witness. “It takes a very long time to study, at least 8 to 10 years just to get in the door as a performer,”

Ms. Qiao said. “And the whole thing is very slow. It’s not like a movie, and right now people want things to be fast. That’s why we’re losing the young crowd.” Still, there seems to be no shortage of students. Young people start their training at 11, going to one of the several Beijing opera academies around the country. “Children really like it,” Ms. Qiao said.

“Another reason is that some parents love it, and they want their children to learn it, even if they’re not thinking about having them become professionals.” Given that Beijing opera is fading in popularity, especially among the younger generations, it seems strange that so many young people would want to go through rigorous years of training.

“It’s such good training that the students can go in almost any direction even if they don’t end up in the opera,” Ms. Liu said. “A lot of our students end up on television or in the movies,” she added. “There are a lot of martial-arts movies, and our students are all good at martial arts.

” The Chinese Ministry of Culture lavishly subsidizes the opera. “Everybody’s doing their best to keep this as a cultural treasure, whether people go to see it or not,” Ms. Qiao said.