During low-dose adrenaline infusion, platelet count, platelet size, plasma ß-thromboglobulin (BTG) and forearm vascular resistance (FVR) were measured in twelve 40-year-old men with mild, untreated hypertension. The average platelet count increased from 195 to 226 x 109/l (P< 0.001), platelet size from 7.31 to 7.53 x 10-15/l (P< 0.01), BTG from 0.61 to 1.08 nmol/l (P< 0.02) and FVR decreased from 97 to 58 (arbitrary units;P< 0.001) during the infusion. The change in platelet count reflects splenic release of platelets, the change in plasma BTG reflects platelet release reaction, while the reduced FVR reflects vascular smooth muscle cell relaxation. In 11 normotensive men aged 40 years, platelet count increased from 187 to 201 x 10g/l (P< 0.01) during an equal low-dose adrenaline infusion. This increase in platelet count is significantly less than in the hypertensive group (P< 0.01). There was statistically no significant change in platelet size, BTG or FVR in the normotensive group. Arterial adrenaline rose from 0.5 to 2.5 nmol/l in the hypertensive and from 0.5 to 2.4 nmol/l in the normotensive group. A third group of 12 normotensive men received saline infusion: neither platelet parameters nor FVR changed in this group. Thus, a small and equal dose of adrenaline elicited a greater increase in platelet count, an enhanced platelet release reaction and a more pronounced forearm vasodilation in hypertensive than in normotensive subjects