Platelets take up and store noradrenaline and adrenaline in proportion to plasma concentrations and may, therefore, provide an integrated index of sympathoadrenomedullary arousal. Plasma and platelet catecholamine concentrations were measured in 290 European and 155 Asian subjects, and were related to blood pressure, insulin levels, and social class. Plasma noradrenaline concentrations correlated with systolic and diastolic blood pressure in both ethnic groups (r = 0.20-0.30), and these levels were significantly elevated in hypertensive subjects (P < 0.003). In Europeans, but not in Asians, these relationships remained significant after adjusting for age and gender, and were independent of the relationship between insulin and blood pressure. There were much weaker relationships between platelet catecholamine concentrations and blood pressure, which were dependent on age and gender as covariates. In neither ethnic group was there any relationship of either plasma or platelet catecholamine concentrations with insulin concentrations, body mass index, or glucose intolerance. European subjects in manual occupations had significantly higher concentrations of plasma noradrenaline than those with nonmanual occupations. The findings suggest a significant role for the sympathetic nervous system in the determination of blood pressure, but do not provide evidence for a connection between hyperinsulinemia and sympathetic nervous system activity. The role of platelet catecholamines as an epidemiological marker of stress has not been supported.