The purpose of this study was to determine the effects of chronic administration of nicotine on red blood cell (RBC) velocity in single capillaries of the mesentery. Ten male rats consumed an average of 2.38 mg nicotine/kg/day, an approximate '2 pack' equivalent, in their drinking water for a period of 15-20 weeks. At the end of this time RBC velocity was measured in single capillaries of the mesentery. The data were compared to RBC velocities measured in a group of matched control rats. Red cell velocities in the control and nicotine groups averaged 1.22+/-0.23 and 0.59+/-0.10 mm/sec SEM, respectively. These values were significantly different (p less than or equal to 0.01). It is concluded that chronic administration of nicotine to rats in levels equivalent to tobacco smoking reduces resting capillary blood flow in the mesenteric circulation as indicated by differences in erythrocyte velocity.