In 11 healthy human subjects, cutaneous red cell flux in the thenar eminence of the left hand was measured by a Laser Doppler meter, while the right hand was transferred sequentially from water at 33 degrees C, to water at 16 degrees C for 5 min, to water at 33 degrees C (1st experimental run) and then to water at 16 degrees C for 2 min (2nd experimental run). Red cell flux decreased from 33 +/- 10 (mean +/- SEM) to 19 +/- 5* perfusion units (pu) by the end of the 2nd min of cooling (* P less than 0.05 paired t-test), but had returned to 27 +/- 8 pu by the 5th min. Arterial pressure, measured by sphygmomanometry from the left arm at the 5th min (1st run) and 2nd min (2nd run) did not change, indicating the changes in red cell flux reflected changes in cutaneous vascular resistance. By contrast, when these experiments were repeated with a cuff on the right arm inflated to 56-62 mmHg to impede venous outflow from the cooled limb, there were no changes in red cell flux in the contralateral hand, nor in arterial pressure. Occlusion per se had no effect on red cell flux. Thus, we propose that cooling one hand evokes a short-lasting reflex constriction in the cutaneous circulation of the contralateral hand which is mainly mediated by core thermoreceptors affected by a reduction in blood temperature, rather than by peripheral thermoreceptors in the cooled hand. The contrast between this proposal and views based on earlier studies is discussed.