Not long ago, Chester Petranek was engaged in a favorite pastime, something he's been doing since 1946. He was scowling and barking orders at a bunch of high school kids.

"At the tip!" Petranek shouted from the front of a crowded classroom. "Now work your way down. Build it, build it. Tip! Tip! Tip!" The first and second violins of the Montgomery County Youth Symphony, hammer and tongs at a Handel concerto, bowed to Petranek's wishes--that is, at the tip, where horsehair skirting catgut will sometimes sound airy. After a few bars, the conductor signaled silence, tapping his baton on the lectern. He pensively pulled his beard.

"Well," he sighed at last, allowing himself a grin, "it's coming." A few weeks later, the orchestra sailed through Handel's "Concerto in D" like seasoned pros in the Kennedy Center's Concert Hall. There was a thunderous ovation. "And these are all just high school students, folks," the master of ceremonies said.

"It's the best youth orchestra in the whole area," the conductor confided, baring his bias. "Possibly, in the whole United States."

The Montgomery County Youth Symphony, capping off its 36th season tonight with a free concert at the National Institutes of Health auditorium, gives aspiring artists a chance to shine and audiences a chance to discover fledgling talent.

By far the oldest such ensemble in town--it predates by decades the D.C. Youth Orchestra and the Northern Virginia Youth Symphony, two comparable groups--it is also among the most venerable this side of the Atlantic. What's more, it has flourished under the same baton since the first swooping downbeat.

"I expect to be doing this till they carry me off the stage," vows Petranek, a 66-year-old chain smoker with salt-and-pepper whiskers. He came to Montgomery County, from a sergeant's billet directing the band at Walter Reed Army Hospital during World War II, to put together an instrumental music program for the county's public schools.

In the bargain, he also started the orchestra, which has become an established training ground for the big leagues of music. A sampling of Petranek's alumni, of whom today there are thousands, includes a string bass player in the Philadelphia Orchestra, a violist in the Chicago Symphony, a clarinet player in the Cleveland Symphony, two flutists and a fiddle player at the Metropolitan Opera, a trumpet player in the Bayreuth Symphony and a fiddle player in the Dutch Radio Orchestra, and still others playing professionally from Houston to West Berlin.

Lewis Lipnick, a bassoonist and contrabassoonist for the National Symphony, recalls his youth orchestra days as "probably the most important musical experience I ever had. It gave me some discipline and exposed me to real music." Yet he suffers pangs of remorse whenever the National Symphony strikes up Beethoven's Emperor Concerto. "I totally bombed out," he says of a bungled bassoon solo 20 years ago in a packed high school gym. "My God, even now, I don't even want to think about it. I remember I just wanted the earth to open up under me and everything to fall on top of me."

What happened instead, as with an extraordinary number of others who joined at a tender age, is that Lipnick went on to a thriving career in music.

These days, the group gives kids a first taste of symphonic repertory and lets them play in an ensemble of unusually high quality.

"It's got to be the best group I've ever played in," said 10th-grader Jim Bryla, a first-year clarinetist. "Even the worst player in that group is so much better than the average high school player. And it's the good groups that really keep you growing."

But the orchestra also acquaints the kids with some of the agonies of musicianship. For instance, would-be players must audition for their seats--a process occasionally strained by Sturm and Drang. For a week last fall, Petranek hunkered down in a high school music room with several packs of cigarettes and a welter of score sheets, there to pass judgment on more than a hundred candidates. Outside in the corridor: a gaggle of nervous kids and more-nervous mothers; in a practice room down the hall: a cacaphony of flutes and fiddles; behind closed doors: the moment of truth.

Fourteen-year-old Deborah Satinsky, a ninth-grader from Potomac, ventured into Petranek's lair, toting a violin. He stood up to shake hands, they returned each other's nod and he peppered her with questions: "Who's your teacher? How old are you? What've you brought to play?" He pointed her to the music stand.

"Relax," he ordered, and she snapped to attention. Then, as the conductor sat stonily marking a score sheet, she rushed through an A-flat scale, and, loosening up, played the third movement of a violin concerto by de Beriot. Petranek let her get halfway through. "Okay, that's enough," he said, and winked at a visitor. The piece had been well-prepared. "Let's do some sight reading," he said, his tone now full of cheer--in contrast to Satinsky's lingering look of distress. He handed her some sheet music, the Handel concerto, and called for a few choice bars. "It goes about like this," he said, tapping his foot as she counted under her breath. When he stopped tapping, she raised her bow. "Take some time, look it over," Petranek said expansively. She launched into the Handel, and Petranek looked down at his score sheet again, hardly grimacing at the sound of a misplaced note. "That's it," he said when she'd finished her measures. "Thanks for coming. We'll let you know." Deborah Satinsky shuffled uncertainly out of the room.

"That," Petranek volunteered as soon as she had gone, "is one very talented kid. That's the kind of audition I like to start my day with." Later she would take her place among the first fiddles.

Another hopeful, a 10th-grade flutist, was not so lucky. He already was hyperventilating when he arrived. "Remember my brother? He was a tall skinny guy?" he asked Petranek between short breaths. "No, I don't think so," came the reply. Then the young man dropped his music on the floor, offered a copy to Petranek ("Don't worry, I know it," the conductor said with a dismissing swat), paced around the room and pleaded for more time to study the sight-reading exercise. "But what were you just doing?" the conductor challenged. After Petranek furiously beat time on the table, the youth literally blew away his prospects. "Thank you," Petranek said, and the boy fled the room near tears.

"Most kids just don't prepare the music," the maestro groused later. "These auditions get the kids on the ball psychologically. It's really an uphill struggle to get a lot of them to work."

Among some of today's hundred-odd orchestra members, as among a few alumni, Petranek has a reputation as a tough customer--not, in any case, a fellow to be trifled with. Once a week, he pushes the kids through two solid hours of rehearsal, a sometimes grueling affair that challenges them with the likes of unexpurgated Brahms, Beethoven, Borodin, Wagner, Dvorak, Tchaikovsky, Mahler, Barber, Stravinsky, Mozart and maybe a little Haydn.

"He really takes some getting used to," said Jim Bryla, voicing the age-old plaint of players about conductors. "When I first started, I was sure he totally hated me."

Youth orchestra alum Danny Orbach, now a violist at New York's Metropolitan Opera, recalled, "Chester wasn't a nasty guy, but he insisted upon discipline. That was a place for training."

And Janet Piez, a contract oboist about town who played under Petranek in the late 1960s, said, "I guess I viewed him very much as I view conductors today. That is, from a distance and with suspicion." She giggled. "Chester always treated us like the professional musicians we weren't. I liked him for that." The feisty Piez had several run-ins with Petranek over the nuances of interpretation. The arguments usually ended, she recalled, when he tossed her out of rehearsal.

"The kids have to learn that I'm the conductor, and that the conductor's in charge," Petranek said. "That's the way it is in the professional world. This is very serious in terms of their futures."

The symphony, a private nonprofit group for which kids pay $50 each to join, has incalculable effect on the music scene in Montgomery County, which, like few other affluent areas in the United States, produces professional musicians far out of proportion to its size. With two orchestras for younger players aside from the high school ensemble--the Prep, for grade school kids, and the Junior, for junior high students--the organization influences what gets taught in private lessons.

"The youth orchestra kind of predetermines what some of the serious students will study," said Mark Ellsworth of Ellsworth Studios, a large music emporium in Bethesda. "They have established norms and required materials, and in many cases, that's what you'll have to teach those kids."

Petranek, who accepts a small honorarium to cover his travel expenses to the weekly rehearsals, leaves the orchestra's nuts and bolts to parents, some of whom, like Malcolm Halliday, the principal of Brookhaven Elementary School in Rockville, played under his baton long ago.

"It's very much the same now as when I was in it," said Halliday, a string bass player who graduated from Montgomery Blair High School in 1954, and whose 14-year-old daughter Hildy currently plays fiddle for the Prep Orchestra. "Chester's still a man who cares both about kids and about music. Though I'd say the kids seem a bit more accomplished today."

At the rehearsal, Petranek was still yelling as the night drew to a close. "Too loud! Too loud! Diminuendo, celli!" he shouted during a Dvorak symphony, piercing the air with his baton. "You get to a half note, and bleccch." He grimaced theatrically, provoking titters from the cellos. "You keep that up and I'm gonna put you in the trumpet section."

A few minutes later, the rehearsal over, he was standing in the hallway, still keyed up and dancing on the balls of his feet. He turned to a visitor. "Aren't they great?" he asked.