Billy Carter, the president's younger and often outspoken brother, was admitted to Sumter County Hospital this morning for treatment of bronchitis, according to his physician, Paul C. Broun.

Carter had been running a "lowgrade fever" and had been experiencing breathing difficulty for some time, but there was no evidence of pneumonia, Broun said.

Carter, 41 and the father of six, was said to be resting in a private room with only immediate family permitted to visit or call him. "He's tired and he's got bad bronchitis," Broun said in a telephone interview.

Broun said he put Carter on antibiotics and had him taking "breathing treatments to help him cough up any mucus in his lungs." He said he had no idea when Carter would be released from the hospital.

Broun, a close friend of Billy Carter, said he had encouraged Carter, a chain-smoker, to stop smoking, but was unsure whether his patient would take his advice.

Carter was last hospitalized in April 1978 to recuperate from exhaustion. "He was getting run down from all the traveling," said Broun.

Billy Carter's wife, Sybil, was released Thursday from an Atlanta hospital, where she was being treated for phlebitis.