NEW YORK, AUG. 7 -- Mayor Edward I. Koch has suffered a "tiny" stroke from a disorder in a small artery to the brain but is "basically healthy" and should be released from the hospital by Sunday, his doctor said today.

Dr. J.P. Mohr, chief of the stroke center at Columbia Presbyterian Medical Center, told reporters that "the injury to the brain is very small" and that the disorder was "neurologically equivalent to breaking his toe."

Concern for the health of the 62-year-old Democrat, this city's dominant political figure, intensified just a day after doctors assured the public that Koch's symptoms had subsided and that he was out of danger. Koch was taken to Lenox Hill Hospital on Manhattan's Upper East Side Thursday morning after complaining of slurred speech, nausea and dizziness.

This morning, Koch was transferred by ambulance to Columbia Presbyterian's Neurological Institute on West 168th Street after the slurred speech and dizziness recurred. Columbia has more sophisticated diagnostic equipment.

Dr. Anthony Mustalish, chief of emergency services at Lenox Hill, said Koch had suffered a transient ischemic attack (TIA), a brief spasm in a blood vessel that cuts off oxygen to part of the brain. If such a condition persists for several hours or more, it is considered a stroke.

Mohr stressed today that the disorder occurred in a tiny artery on Koch's right side, one that helps "control movement of the face, arm and voice on the opposite side. The mayor's left face and left arm are barely affected. His voice is slightly affected."

Brain damage could have been far more serious if a major artery had been affected, but Mohr said advanced magnetic resonance scans showed that blood flow in Koch's major arteries is "entirely normal." He said Koch had not suffered a relapse but a continuation of the disorder that hospitalized him Thursday.

Aides to Koch, who sought to portray him Thursday as alert and making wisecracks, were more somber today. But Mohr was upbeat in his assessment, saying that Koch should be able to resume his duties by late next week.

Given Koch's feisty manner, he said, "I think it's necessary for his health to continue as mayor."

Mohr said he would prescribe that Koch take one aspirin a day, which helps prevent blood clots, and that he watch his diet. High blood pressure, high cholesterol, diabetes, obesity and smoking are among the risk factors associated with stroke.

Dr. David Levy, associate professor of neurology at New York Hospital, said a TIA episode is "a marker as an indicator for stroke" and puts the victim "at greater risk" of suffering a significant stroke later. He put the chances at about 25 percent over five years.

Levy said that if Koch's neurological disorder lasted more than a few hours, "that's potentially fairly serious."

Transient ischemic attacks are relatively common in persons over the age of 40 and are seen more often in men than in women. In recent decades, experts say, the incidence of stroke has declined by 30 to 40 percent, most likely due to better health habits.

Under the city's charter, if Koch were to become incapacitated, City Council President Andrew Stein would become acting mayor, although there are many unresolved questions about who would decide if Koch were incapacitated and when.