The effects of insulin infusion (1.5 u/h and 15 u/h) on finger nailfold capillary diameter, microcirculatory resting flow and microvascular reactivity were studied in eleven Type 1 diabetics. Blood glucose was maintained at pre-infusion values by intravenous glucose infusion as necessary. Venous limb capillary diameter was significantly wider on 15 u/h compared with pre-infusion values (14.3 +/- 4.5 micron pre, 16.9 +/- 5.1 micron 15 u/h, p less than 0.01. Resting blood flow measured by laser doppler flowmetry increased on the low dose infusion compared with pre-infusion values (2.6 +/- 1.7 V pre, 3.3 +/- 1.9 V 1.5 mu/h, p less than 0.05) but fell on high dose infusion (2.5 +/- 1.9 V, p less than 0.02). Dynamic measurement of capillary blood flow velocity in single capillaries suggested that insulin infusion increased the circulatory debt repayment following 60 s arterial occlusion. The high dose infusion caused a significant impairment of the reflex rise in precapillary resistance that normally accompanies venous congestion. It is concluded that insulin has effects on skin microvascular haemodynamics that are independent of the hormone's hypoglycaemic action.