Parasympathetic as well as cardiovascular sympathetic activity is perturbed in the heart of diabetic and obese subjects. The aim of this work was to assess the effect of sympathetic control on variations in skin blood flow in 14 controls, 42 subjects with non-insulin-dependent diabetes and 57 obese subjects without diabetes. All subjects had normal blood pressure and had no other disease state which might modify skin blood flow. Laser-Doppler measurement of skin blood flow on the palmar aspect of the right first finger was performed at rest and during three tests which activate sympathetic tone: deep respiration (R), active orthostatism (O) and Valsalva's manoeuvre (V). In all 113 subjects, there was a negative correlation between age and the standard deviation of blood skin flow at rest (p = 0.001) and rate of blood skin flow decline during R (p = 0.002), O (p < 0.0001) and V (p = 0.005). The standard deviation and slope of decline in blood skin flow during R was lower than the initial levels in controls in 11 diabetes patients and 6 obese subjects and in 15 diabetic patients and 12 obese subjects respectively. The slope of decline in skin blood flow during O was positively correlated with plasma cholesterol (p = 0.002), red cell aggregation index (p = 0.013) and negatively with hematocrit, glucose control and creatininemia. The respective effects of these factors were analyzed in each of the three groups of subjects. These preliminary results suggest that 1) the variation in skin blood flow at rest is regulated by the sympathetic system and the drop in skin blood flow during the three tests activating the sympathetic system decreases with age, 2) sympathetic control of peripheral vasoconstriction is reduced in many diabetic and obese subjects, 3) red cell aggregability and different metabolic parameters and renal function have an effect.