Billy Carter, 51, the younger brother of former President Jimmy Carter whose cantankerous personality and colorful and controversial quips made him a national figure in his own right, died of cancer yesterday at his home in Plains, Ga.

He was diagnosed with inoperable cancer of the pancreas on Sept. 11, 1987, two days after he entered Emory University Hospital in Atlanta. On May 23, he began an experimental treatment at the National Cancer Institute, where he was treated with the drug interleukin-2. Last week, he asked to be sent home from an Atlanta hospital so he could be with his family.

His father, James Earl Carter, and a sister, Ruth Carter Stapleton, also died of pancreatic cancer in their 50s.

Former President Carter issued a statement yesterday morning, saying, "He was very courageous in his last years of life."

A White House spokesman said President and Mrs. Reagan "were deeply saddened by the news and extend their sympathies and prayers to the Carter family."

Billy Carter became widely known to the public in 1976 as the salty, down-home brother of a little-known former Georgia governor who was seeking the Democratic Party presidential nomination. Mr. Carter was often photographed and interviewed near his Plains gas station, where he held court in a carefree and friendly manner. He had pithy and picturesque observations on his world philosophy and tales of his brother's early life.

This picture hid the more complex tones of his portrait. He was not a simple gas station owner: He had taken a marginal family peanut warehouse enterprise and built it into a $5 million-a-year operation, and had numerous successful real estate investments. Although Mr. Carter seemed to bask in his brother's fame, others believed that he had long felt himself unappreciated by some members of his family.

During his brother's presidency, Mr. Carter's public life became increasingly difficult. With the family peanut business in trust, he found himself with time on his hands.

He toured the personal appearance circuit, had a beer named after him, made two trips to Libya and became something of a defender of that country and Arabs in general. He eventually registered with the Justice Department as an agent of a foreign government.

He also became the subject of congressional groups investigating links between Mr. Carter and Carter family businesses and Bert Lance. Lance, who was President Carter's first director of the Office of Management and Budget, left government in the midst of news stories about his tenure as a bank president in Georgia. No wrongdoing by Mr. Carter was discovered.

Mr. Carter eventually was forced to sell much of his property to settle disputes with the Internal Revenue Service. He also was a subject of government groups seeking to find out whether money was illegally diverted from the Carter family businesses to the 1976 Carter presidential campaign. No illegalities were found.

Mr. Carter's popularity waned and his troubles mounted. He acknowledged that he was an alcoholic, or, as he put it, "the world's most public drunk."

He later explained that the radical change in his life style led to his increasing reliance on alcohol. "Instead of working 12 hours a day, I was working three or four hours a day on the celebrity tour and had 15 to 20 hours of drinking."

He entered the Long Beach, Calif., Naval Hospital, where for seven weeks he underwent treatment for alcohol abuse and exhaustion. He said he never drank after leaving the hospital.

In earlier, happier days, he had been the picture of a man who was on top of the world and enjoyed a can of beer or two. It was then that a Kentucky brewer started marketing "Billy Beer." The venture lasted a couple of months. Mr. Carter later joked that one of the reasons he gave up drinking was that his own beer was so bad.

Probably the watershed event in Mr. Carter's popularity involved Libya. Before that, he was widely regarded as the funny, carefree and prankish younger brother to a somewhat dour, humorless and perfectionist elder brother. His Libyan involvement transformed him into a political liability for the president.

The troubles began when he traveled to Libya as the guest of its leader, Moammar Gadhafi, then was host to a Libyan delegation traveling to Atlanta. He criticized then-mayor Maynard Jackson for snubbing his guests and accused the mayor of bowing to pressure from the city's Jewish community. Mr. Carter added, "There's a hell of a lot more Arabians than there are Jews."

These and other remarks led to a storm of protests from U.S. Jews. Mr. Carter denied that he was anti-Jewish in any way, and pointed to close friends and relatives who were Jews, and said that his remark about the number of Arabs was "fact," not evidence of prejudice.

It also came to light that Mr. Carter had secured a Libyan loan for more than $200,000. At his brother's urging, and after the Justice Department brought suit, he registered, under protest, as an agent of the Libyan government.

He later said, "The dumbest thing I ever did was my first trip to Libya."

Though the president did not speak to his younger brother for eight months at the height of Mr. Carter's Libyan involvement, President Carter stood by him in public. The president took issue with some of his brother's statements, told reporters his brother was entitled to his views, and publicly joked about his younger brother's contributions to the brewing industry. President Carter also said his brother was a fine man.

By the time Jimmy Carter lost the 1980 presidential race and returned to Georgia, his brother had left the state. He sold mobile homes in Alabama until returning to Plains in 1986. During the 1988 Democratic presidential primaries, he campaigned for the Rev. Jesse Jackson. In something of the voice of the old Billy Carter, he said, "Every black needs a redneck in his campaign."

William Alton Carter III was the youngest of four children. He grew up in Archery, Ga. As a small child he stuttered. He did badly in schools in which his brother and sisters had stood out. When he was 13 years old, the family moved to Plains, where Mr. Carter's academic woes continued. He graduated next-to-last in his class from his high school.

When he was 16, his father died, and Mr. Carter became something of the man of the family. He ably ran the family business until his brother, 13 years older, could resign his Navy commission and take the business over.

Mr. Carter married his high school girlfriend, Sybil, then enlisted in the Marines. After that, he drove trucks, then attended Emory University before leaving because of failing grades. He worked in the construction industry and was a salesman before returning to Plains in 1964. As his brother became more involved in politics, Mr. Carter became more of a force in the family business. And at that, he was a success.

In his later life, he sometimes said that in some ways he wished he had been allowed to run the family business, but on the whole he enjoyed his time in the national spotlight. He said, "I've been asked, a thousand times, I guess, what I would do if I had it to do over again. And I said, 'Probably the same thing,' because if I had to do it over again, I'd probably screw up worse the second go-round."

In addition to his wife and brother, Mr. Carter is survived by six children: Kim Fuller, 31; Jana Carter, 29; William Alton (Buddy) Carter IV, 27; Marle Usry, 25; Mandy Carter, 20, and Earl Carter, 11; and a sister, Gloria Carter Spann. His mother, Lillian Carter, died in 1983.

WALTER D. ADDISON

Farmer & Real Estate Executive

Walter Dulany Addison, 90, a retired area farmer and real estate executive who served three terms as registrar of wills for Prince George's County from 1954 to 1966, died Sept. 23 at Fairfax Hospital after a heart attack.

Mr. Addison, who lived in McLean, was a native of Glenn Dale. He farmed near there until about 1960. From 1958 to about 1970, he was a partner in the family real estate firm of Addison Realty in Upper Marlboro. He had been a desk officer with the Maryland State Senate from 1929 to 1954, and also had served as a director of the state tobacco authority.

He had served on the vestry of Holy Trinity Episcopal Church in Bowie. He had been a member of the Southern Maryland Society and the Bowie Democratic Club. He was a charter member of the Prince George's County Farm Bureau.

His first wife, the former Edna Matthews, died in 1964. His second wife, the former Marion Holland Mackall, died in 1984. He leaves no immediate survivors.

WILLIAM F. KROENER IV

St. Albans Graduate

William F. Kroener IV, 21, a 1986 graduate of St. Albans School who was a junior at Ohio Wesleyan University, died Sept. 23 at a hospital in Elyria, Ohio, as a result of injuries he received in an automobile accident earlier that day.

A spokesman for the Ohio State Highway Patrol said Mr. Kroener was driving north on state Rte. 58 in Amherst, Ohio, when a car driven by a 19-year-old woman from South Amherst, Ohio, crossed the center lane and hit him head-on.

A passenger in Mr. Kroener's car, John Eddy, 21, of Short Hills, N.J., also died as a result of the accident, and a second passenger, James Riesz, 20, of Bloomington, Ind., is hospitalized in critical condition, police said. The passenger in the second car, a 19-year-old Ohio woman, also was injured, police said. The accident is under investigation.

Mr. Kroener was born in New Haven, Conn., and lived in New Canaan, Conn., before moving to the Washington area in 1982.

Survivors include his parents, Mr. and Mrs. William F. Kroener III, a brother, James M., and two sisters, Mary E. and Evangeline A. Kroener, all of Chevy Chase; a grandfather, Dr. John D. Bibb of La Habra, Calif., and his grandmothers, Barbara M. Kroener of Newport Beach, Calif., and Evelyn R. Bibb of Whittier, Calif.