Because he is still learning about being a circuit judge, Larnzell Martin Jr. made sure to keep quiet and listen. He sat at the bench in Courtroom 202 in Upper Marlboro next to an experienced colleague, Judge William D. Missouri, and took notes with a pencil.

Now and then he looked sideways at Missouri, then glanced down at the defendant, Donnell Petite, who was standing, waiting for Missouri to sentence him to prison. Petite, who turned 18 a few months ago, had no criminal record before last spring, when he and two friends got drunk and hurled five- to 15-pound rocks at motorists on the Capital Beltway.

As he watched and listened and jotted more notes, Martin recalled later, he wondered what sentence he would impose were it up to him. He was unsure. After 12 years as a lawyer immersed in contracts, civil matters and administrative work, and two years on the District Court bench dealing with small disputes and petty offenders, the county's newest Circuit Court judge is not accustomed to serious crime.

But his education is underway.

"I was reviewing the pre-sentence investigation report {in Petite's case}, and that was perhaps the first time I really felt that kick-in-the-stomach sensation, regarding just the weight of the responsibility," said Martin, who was sworn in Dec. 17. He has been sitting with other circuit court judges this month, observing them as part of his orientation.

"I haven't talked with Bill Missouri about the kind of anguish he goes through in a sentencing like that," Martin said. "But I think that's probably going to be the hardest part of it for me."

He was referring, he said, to the difficulty of fashioning sentences, of developing a philosophy. "I personally want to look at some literature that relates to theories of sentencing, so I can get some personal philosophical bearing," Martin said.

It would serve him well in Prince George's, which ranked third among Maryland's 24 jurisdictions, behind Baltimore and Baltimore County, in the number of felony cases filed last year.

"It's very important for a judge to feel comfortable with what he or she does, and the way to obtain that comfort is to be consistent," said Martin, one of 18 circuit judges in the county. "You have to be predictable to a certain degree. If you feel you're doing the right thing the first time around, the second time, the third time, then you'll be comfortable later. It's the comfort of not second-guessing yourself."

For now, though, he watches and takes notes. Studying the pre-sentence report on Petite's background a few weeks ago, before Judge Missouri imposed a 39-year prison term, Martin recalled: "I just thought, 'I'm glad I don't have to do this.' And the reason was, I'm not ready yet."

Martin, 40, grew up in Dallas and studied sociology and anthropology as a college undergraduate in Minnesota. After graduating from Georgetown University Law Center in 1975 he almost became a prosecutor.

The Prince George's State's Attorney's Office had a job opening, Martin recalled recently. He went to Upper Marlboro to apply. "During the course of filling out the application, I said to myself, 'I never wanted to do criminal law.' "

He was sitting alone in the office's waiting area. With no one watching, Martin said, "I ripped up the application and left."

Instead, he chose the county attorney's office, which handles civil, administrative and regulatory matters for the Prince George's government. Hired in February 1977, he specialized in contract law and eventually served as legal counsel to the committee that drafted regulations and awarded the contract for cable television in the county.

After moving to private practice in 1982, Martin returned to the county government in 1984 as deputy county attorney. County Executive Parris N. Glendening appointed him to the top job two years later, making Martin the first black county attorney in Maryland. In 1988, Gov. William Donald Schaefer appointed him to the District Court.

He found the mix of cases in District Court appealing, he said. While administering the law, he found he also was able to act as a sort of social worker, a field in which he once had intended to make a career.

Cases heard in District Court include misdemeanor crimes and such relatively minor civil matters as disputes between landlords and tenants. The cases lend themselves to mediation more easily than Circuit Court matters, Martin said.

Circuit Court is where felony cases are handled. When the General Assembly last year approved an 18th judgeship for the court in Prince George's, Schaefer chose Martin over four other candidates.

It takes some getting used to. "In District Court, because of the nature of the cases, it's not as grand," Martin said. "There's not much public interest in what happens. But in Circuit Court, when you sentence someone, particularly in high-profile cases, it's like everyone in the world is staring at you. And I've got to make sure I'm not driven by that stare . . . .

"It's just a matter of trying to develop in myself a sufficient philosophical anchor," he said. "If I do something, and someone calls me up and asks me why, then I want to be able to explain what I did with confidence. I want to be able to say that if I had it to do over again, I would."