To test for a diurnal difference in the vasoconstrictor control of the cutaneous circulation, we performed whole body skin cooling (water perfused suits) at 0600 hours (AM) and 1600 hours (PM). After whole body skin temperature (T_sk) was controlled at 35°C for 10 minutes, it was progressively lowered to 32°C over 18-20 minutes. Skin blood flow (SkBF) was monitored by laser Doppler flowmetry at three control sites and at a site which had been pretreated with bretylium by iontophoresis to block noradrenergic vasoconstriction. After whole body skin cooling, maximal cutaneous vascular conductance (CVC) was measured by locally warming the sites of SkBF measurement to 42°C for 30 minutes. Before whole body skin cooling, sublingual temperature (T_or) in the PM was significantly higher than that in the AM (P < 0.05), but CVC, expressed as a percentage of maximal (%CVCmax), was not statistically different between AM and PM. During whole body skin cooling, %CVCmax levels at bretylium treated sites in AM or PM were not significantly reduced from baseline. In the PM, %CVCmax at control sites fell significantly at T_sk of 34.3 ± 0.01°C and lower (P < 0.05). In contrast, in the AM %CVCmax at control sites was not significantly reduced from baseline until T_sk reached 32.3 ± 0.01°C and lower (P < 0.05). Furthermore, the decrease in %CVCmax in the PM was significantly greater than that in AM at T_sk of 33.3 ± 0.01°C and lower (P < 0.05). Integrative analysis of the CVC response with respect to both T_or and T_sk showed that the cutaneous vasoconstrictor response was shifted to higher internal temperatures in the PM. These findings suggest that during whole body skin cooling the reflex control of the cutaneous vasoconstrictor system is shifted to a higher internal temperature in the PM. Furthermore, the slope of the relationship between CVC and T_sk is steeper in the PM compared to that in the AM.