Reflex vasodilation is attenuated in aged skin during hyperthermia. We used laser Doppler imaging (LDI) to test the hypothesis that both the magnitude and the spatial distribution are altered with aging. LDI of forearm skin was compared in 12 young (19 - 29 yr) and 12 older men (64 - 75 yr) during supine passive heating. Additionally, iontophoresis of bretylium tosylate was performed in a subset of subjects to explore the involvement of sympathetic vasoconstriction in limiting skin blood flow. Passive heating with water-perfused suits clamped mean skin temperature at 41.0 ± 0.5°C causing a ramp increase in esophageal temperature (T_es) to 38.5°C. LDI scans were performed at baseline and at every 0.2°C increase in T_es. LDI at bretylium and control sites was identical, suggesting no influence of noradrenergic vasoconstriction. Forearm vascular conductance (venous occlusion plethysmography) was reduced in the older men (P 0.001) at every elevated T_es. Mean cutaneous vascular conductance of the scanned area was reduced in the older subjects at 0.2°C T_es 0.8°C. Early in heating (0.2°C T_es 0.6°C), older men also responded with a reduced vasodilated area (P 0.05) implying a slower recruitment or filling of skin microvessels. These results indicate that both the area of vasodilation and CVC within the vasodilated area are reduced in aged skin during early passive heating, but only CVC is reduced at T_es = 0.8°C.