White House press secretary James S. Brady has developed pneumonia in the lower lobe of his left lung and is receiving intravenous antibiotics, according to doctors at George Washington University Hospital.

Brady had episodes of fever in the range of 102 to 105 degrees on Tuesday and Wednesday nights, prompting tests to pinpoint the cause, according to Dr. Dennis O'Leary, the hospital's dean for academic and clinical affairs. Each time his temperature returned to normal in the morning.

Initially, the only abnormality doctors discovered was a high white blood cell count, O'Leary said. The result of a chest X-ray, spinal tap, cross-sectional X-rays of the brain and other tests done Wednesday were all normal, and doctors began to suspect the fever was a reaction to a medicine, a problem Brady has had twice before during his hospital stay.

But O'Leary said that yesterday around noon, a neurosurgery resident examining Brady heard crackling noises in his left lung. Brady had no cough or difficulty breathing, but the surgeon ordered a repeat chest X-ray, and this time it revealed pneumonia.

O'Leary said the discovery relieved Brady's doctors, who were worried about the possibility of a more serious infection in the brain or sinuses, which were damaged by his bullet wound in the March 30 shooting attack on President Reagan. "We feel actually good about this, because this is . . . a highly manageable problem," he said. "It could have been in a much worse place."

He said the pneumonia was apparently unrelated to a complication Brady suffered earlier this month, in which blod clots from his legs traveled to his right lung. He said no clots lodged in the left lung, and the problem has not recurred.

Wednesday, doctors started treatment with two antibiotics effective against a variety of bacteria, after performing cultures of Brady's blood, urine, nose and spinal fluid, trying to locate the infection. But since Brady had no cough, they did not culture his sputum, and O'Leary said it may be difficult now to determine what kind of bacteria are causing the pneumonia.

"The concern is that we don't have a bug," he said. "We don't know what we've got." Since Brady may be infected with hospital bacteria resistant to some drugs, O'Leary said he may continue to receive more than one antibiotic.

He said Brady will stop going downstairs for physical therapy until his fever is under control, although therapists will work with him in his room. Despite the intermittent fever, O'Leary said Brady does not appear to be gravely ill. "I wouldn't say he's quite as dynamic as he's been, but he doesn't look like someone with a bad infection," O'Leary said.