At 6 feet 4 1/2 inches, David Marsden is no shrimp, but even he was a tad intimidated by one of his inmates at the Fairfax County Juvenile Detention Center. The youth had ripped a piece of metal from the ceiling light and was hammering it, and his fists, against the door of his cell.

"He was six feet, three inches and 230 pounds of absolute sculpted fury," recalled Marsden, who this week ends his 17-year tenure as superintendent of the 121-bed facility. Marsden negotiated through the door with the teenager, who finally agreed to be led away in handcuffs.

That was a year ago, one small highlight--or lowlight--in Marsden's lengthy career at the detention center, which is behind the county courthouse in Fairfax City. Tomorrow, Marsden will leave such moments behind for the more predictable world of lobbying and consulting, and with more than a hint of nostalgia.

"It's been nothing but fun," he said in a farewell interview last week at the single-story, red-brick detention center.

Despite the sadness in many of the lives that came before him and the challenge of keeping cool when youths use every verbal weapon--and sometimes physical ones as well--against their caretakers, Marsden said he enjoyed what he did.

"The fun part has been working with people toward common goals, seeing that we're able to get kids through their worst crisis and have them come out of it better able to deal with the problems that they're having," he said.

The center is a temporary home to an increasing number of youths and teenagers under 18, male and female. Before 1982, Fairfax housed minors awaiting trial or sentencing in a facility it shared with Alexandria. But juvenile crime in Fairfax more than kept pace with the county's growth, and a 33-bed facility was expanded first to 55 beds and then to 121 last year.

With the expansion came a new program: Judges, who previously could sentence offenders either to jail time or to parole while living at home, now have the middle ground of sending them to the juvenile detention center for education and therapy. Early indicators point to the program's success, Marsden said.

"It's no panacea, but it seems to be filling a real need," he said.

Marsden attributes the jump in crime to the proliferation of gangs and an increasing number of immigrant families where parents struggle with multiple jobs while their children go largely unsupervised.

The result, he said, has been some heart-tugging stories--and some abusive behavior from children.

"They'll insult you, they'll call you names, they'll hurt you where they think you're weak," he said. In turn, staff members have to be careful not to be abusive themselves, Marsden said.

"It's almost like the Hippocratic oath--you try not to do any harm," he said.

Marsden's assistant and likely successor, Madeline Arter, said she has learned from her boss.

"He has a forgiving style of leadership," she said. "He has the ability to encourage his staff, and he encourages kids as well."

Juvenile inmates live in various sections of the facility, each complete with cells, group rooms and classrooms. No one staff member ever has an eye on all the youths at once, Marsden said, but an adult supervisor is within earshot of nearly every word that is spoken.

"We can run this building a whole lot better with our ears than our eyes," he said.

The veteran superintendent is leaving because, at 51, his 29 years with Fairfax County have earned him a pension. "I was a child bride to the system," he joked.

He's leaving, too, because he wants to stop bothering with administrative work and instead lobby state government about juvenile justice policy.

And Marsden is leaving because, as he put it, "It's time for other people to have some fun."

CAPTION: Marsden, in the center's technical vocation training room, says the program's education and therapy have been successful with juvenile offenders. "It's no panacea, but it seems to be filling a real need," he said.

CAPTION: After 17 years, David Marsden is stepping down as superintendent of the 121-bed Fairfax County Juvenile Detention Center.

CAPTION: David Marsden stands in a cell in the detention center's girls' wing. He plans to lobby the state government about juvenile justice policy.