The effect of cigarette smoking on glycemia was investigated in 26 diabetic patients and 24 normal controls, all smokers. Using the method with ortho-toluidine à jeun, blood glucose levels were determined before smoking and 15, 30 and 60 minutes after the smoking of two cigarettes. Both groups showed an increase of glycemia following smoking, more marked, however, in the group of diabetics. In 16 cases the experience was repeated once more, and an even higher increase of the blood glucose values was recorded as a result of the new nicotine charge. Conversely, no glycemia rise was noted after smoking nicotine-free cigarettes nor after smoking tobacco cigarettes but without inhaling the smoke. The increase of glycemia after smoking is assumed to be due to the mobilization of catecholamines and the stimulation of STH and cortisol production. This reaction seems to be more marked in diabetics than in metabolically normal subjects.