David L. Bazelon, 83, former chief judge of the U.S. Court of Appeals for the District of Columbia Circuit and a pioneer in the application of the principles of psychiatry to criminal law, died of pneumonia Feb. 19 at his home in Washington. He had Alzheimer's disease.

In more than three decades on the court, Judge Bazelon gained a national reputation as a champion of a variety of liberal causes and a defender of the rights of the accused. Often controversial, his decisions and opinions made him a folk hero to some, but he was anathema to many conservatives who argued vehemently that he was soft on crime and that his rulings only turned criminals loose to strike again.

He was probably best known for a 1954 decision that became instrumental in revising the legal definition of insanity. That decision broadened the century-old notion that in order to be acquitted on grounds of insanity an accused criminal had to be unable to distinguish right from wrong or that his crime had to have been the result of an "irresistible impulse."

Judge Bazelon introduced the concept that an accused criminal was also not responsible for his actions if they were the product of a mental disease or defect. It became known as the Durham rule after Monte Wayne Durham of Washington, a convicted burglar with a history of mental disorders who was confined to a mental hospital rather than jailed as a result of Judge Bazelon's decision in the case.

Throughout his career, Judge Bazelon believed passionately that the mission of the criminal justice system should be rehabilitation, not punishment. "If we were not so set on punishing the offender for the sake of punishment, if we did not justify this practice by reference to its deterrent effect, we could understand that rehabilitation lies at the spiritual heart of any vital moral system," he wrote in a 1960 article in Atlantic Monthly.

He sat on dozens of boards and commissions, wrote voluminously and gave scores of speeches in which he argued that the social and psychological situation of the accused should always be considered in a criminal case. "You can't have criminal justice without social justice," he said in a 1979 interview with The Washington Post.

Retired Supreme Court Justice William J. Brennan Jr. called him a pioneer in many areas of the law who was "one of the most important judicial figures of this century."

At 40, Judge Bazelon was the youngest man ever named to the federal appellate bench when President Harry S. Truman appointed him to the Court of Appeals in Washington in 1949. He was chief judge from 1962 to 1978, during an activist period when his court was said to be second in national significance only to the U.S. Supreme Court and was viewed by many as the nation's most liberal and innovative judicial body.

Judge Bazelon wrote or participated in decisions that ranged from ordering a limitation on cross-ownership of broadcasting stations and newspapers in the same city to a striking down of almost everything the government had done to prosecute 12,000 people arrested during the May Day antiwar demonstrations in Washington in 1971.

In a 1957 decision -- later upheld by the Supreme Court -- he ordered the rape conviction of a Washington man, Andrew Mallory, set aside because a confession had been extracted before Mallory had been presented to a magistrate.

The influence of many of Judge Bazelon's opinions extended far beyond his own courtroom and affected the thinking of a generation of lawyers and educators.

For 13 of his most productive years on the court, Judge Bazelon sat on the same appellate bench as Warren E. Burger, who in 1969 became chief justice of the United States. The two were always proper and correct toward each other in the courtroom, but privately the liberal Judge Bazelon and the conservative Burger became bitter antagonists.

Burger's ascendancy coincided with a waning of Judge Bazelon's influence in the legal community. During the 15 years that Earl Warren was chief justice and a liberal coalition dominated the Supreme Court, lawyers often cited Judge Bazelon in their briefs when he was on their side; afterward, his name was still cited, but as a reason to overturn rulings with which he seemed to concur.

In 1978, Judge Bazelon stepped aside as chief judge, and a year later he assumed the semi-retired status of senior judge although he still continued to hear cases.

David Lionel Bazelon was born Sept. 3, 1909 in Superior, Wisc., the youngest of nine children. His father, the proprietor of a general store, died when the future judge was 2 years old, leaving the family virtually penniless. When he was still a child, his family moved to Chicago. There, he attended the city's public schools, the University of Illinois for one year and then Northwestern University, where he received his law degree in 1931. To pay for his education, Judge Bazelon worked as a store clerk and a movie theatre usher.

He was an assistant U.S. attorney and later a lawyer in private practice in Chicago, where he was active in Democratic Party politics. In 1946, he moved to Washington to take a job at the Justice Department. There he was in charge of the lands division and later the office of alien property before Truman named him to the Court of Appeals in October of 1949.

From the beginning of his career on the bench, Judge Bazelon demonstrated an intense interest in the insanity defense in criminal cases, a defense that for more than a century had been essentially limited to the M'Naghten Rule, which required a jury to consider only whether the accused knew his act was wrong. In some courts, an accused criminal also was relieved of criminal responsibility if he could prove his act was the result of an "irresistible impulse" that he could not control because of a mental disorder.

In the Durham decision, Judge Bazelon broadened the insanity defense to include the concept that an accused person was not responsible if his act was the "product of a mental disease or defect." That rule was never widely adopted by other courts, and it was abandoned by the Court of Appeals here in 1972, but it prompted wide-ranging debate on the role of psychiatry in criminal law. It also drew a storm of protest from law enforcement officials.

In dropping Durham for a tighter standard that required a defendant pleading insanity to show that a mental disease or defect prevented him from understanding that he acted wrongly or that he was unable to obey the law, the Court of Appeals said Durham had given the psychiatrists too much leeway.

Long afterward, Judge Bazelon told an interviewer that all he intended was "to open the door to information -- period. The door had been closed shut." In a 1960 article in the Saturday Evening Post, he had written that his "underlying purpose was to unfreeze the expanding knowledge of psychiatry as it could be applied to the law."

Although a firm believer in applying psychiatry to criminal law, Judge Bazelon was also acutely aware of the profession's limitations and shortcomings. Criminal behavior, he told a gathering of correctional psychologists in 1972, may be an "inevitable byproduct of our society's social and economic structure." Criminals, he said, don't necessarily need therapy.

"Poor, black offenders are not necessarily sick. They may simply be responding to an environment that has impoverished them, humiliated them and embittered them," he said.

Despite such reservations, Judge Bazelon served on the board of directors of the Washington School of Psychiatry, the advisory committees for the Center for Law and Psychiatry of the Psychiatric Institute Foundation and the National Association of State Mental Health Program Directors. He was a chairman of the task force on law of the President's Panel on Mental Retardation in 1961 and 1962. He was chairman of the advisory committee of the D.C. model school division's Cardozo Project from 1964 to 1966, and he received several lectureship awards from psychiatric organizations.

Stern and forbidding in court, Judge Bazelon was a harsh and demanding taskmaster with law clerks and others who worked for him, and was sometimes brutally frank about their faults. At the same time, he could be generous and genuinely concerned about their personal lives. In private company, he was warm and gregarious.

Survivors include his wife of 56 years, the former Miriam M. Kellner, who lives in Washington; two sons, James A., of San Diego, and Richard L., of Philadelphia; a brother, Gordon, of Palm Springs, Calif.; a sister, Anne Safer of Milwaukee; and five grandchildren.