Two Washington cardiologists reported yesterday that last January they had advised Rep. Goodloe Byron, a marathoner who jogged almost every day, to stop running. Byron died almost instantly last Wednesday of an apparent heart attack as he neared the end of a 15-mile run.

The findings of the test nine months ago revealed "a severe abnormality" in Byron's heart, the doctors said. According to Dr. Robert L. Blynn, a Chevy Chase cardiologist who administered the test, not enough blood was getting to the heart when Byron took light exercise during a test Flynn's office.

Flynn sent his findings to Dr. Samuel M. Fox, director of the cardiology exercise program at Georgetwon University Medical Center. Fox agreed with Flynn that it was "strong and clear" that Byron, 49, should stop running.

More exhaustive tests were scheduled, Fox said. But Byron's heavy congressional workload, plus a tough primary fight, consumed so much of his time that he never went to see Fox. On several occasions, Fox said, Byron was "within hours of coming in."

Because of the alarming test results - which paralleled the results of tests the Congressmen took in 1974 - both doctors advised their patient to cancel his plans for the Boston Marathon, about 10 weeks away. Byron entered anyway, even though the application form for the race urges runner to consult a physician before competing. He completed the race in more than four hours.

At the same time the two cardiologists gave the details of Byron's rejecting their counsel, they told of their "sympathetic understanding of his reaction."

In a moving, and at times poignant, meeting in the living room of the Byron home, with family members and several guests on hand, the congressman's family and doctors both painted a picture of a man who conscientiously watched his weight and diet and who read much of the literature on running and heart disease.

The latter was never far from his mind. Byron's family had a history of heart problems: a brother died at 32 of a coronary, an uncle at 42, aslo of a heart attack, and an older living brother has had two heart bypass operations.

Because of this history, said Flynn, Byron's running "most likely prolonged his life. How long I can't say." Over the years, Byron had run about a dozen marathons, as well as several 50-mile events.

Byron's widow Beverly, who last week succeeded him as the Democratic candidate for ongress in Maryland's 6th District, stressed emphatically yesterday that running was a healthy influence in Byron's life.

"If Goodloe had not kept him in top physical condition, he would have died much sooner," she said. "He had a strong commitment to preventive medicine." The Byrons' three children said yesterday that they shared his view, adding that they felt they would have lost their father much sooner if he had not been a runner.

Running, they said was his way of staying alive.

At the time of his death, which came as he was jogging along the Chesapeake and Ohio Canal with a staff aide, Byron was planning to run in a marathon in Washington in early November.

The early form for his event also urges runners not to participate unless they have checked with their physician.

In the Sept. 12 Democratic primary, when most church leaders split their support between Tucker and Washington, Williams supported Tucker. Yesterday, with Barry sitting across from him in the pulpit, clapping his hands to the gospel music and raising his hands in prayer, Williams urged support for Barry.

"God bless this young man," the gravel-voiced, 71-year-old Williams told his congregation, pointing his white-robed arm at Barry and lifting his head toward the sky. "I want to say to other preachers, if you get around him and talk to him he'll be able to do what he's supposed to do."