1. Laser Doppler flowmetry has been used to study changes in cutaneous erythrocyte flux produced in the hand (i) on successive immersion of the contralateral hand in water at 20 degrees C (cold test) and then in water at 0-4 degrees C (cold pressor test), and (ii) by mental arithmetic. 2. In 11 subjects, placing the right hand in water at 20 degrees C for 2 min induced a significant decrease in cutaneous erythrocyte flux in the contralateral hand and a significant fall in mean arterial pressure. Cutaneous vascular resistance, calculated as arterial pressure/cutaneous erythrocyte flux, showed no significant change. Thus, the decrease in erythrocyte flux was apparently due to a fall in perfusion pressure. 3. Subsequent immersion of the right hand in water at 0-4 degrees C for 2 min caused a significant decrease in erythrocyte flux in the contralateral hand and a significant rise in mean arterial pressure. It is concluded that the cold pressor response evoked from one hand elicited a substantial reflex vasoconstriction in the skin of the other hand; accordingly, calculated cutaneous vascular resistance increased significantly. 4. Eight subjects performed mental arithmetic for two periods of 2 min separated by a rest period of 2 min. By the end of the second minute of each period of mental arithmetic there was a significant decrease in erythrocyte flux. Mean arterial pressure increased significantly in the first period only, but calculated cutaneous vascular resistance increased in both periods, consistent with cutaneous vasoconstriction.(ABSTRACT TRUNCATED AT 250 WORDS)