We have used high-performance liquid chromatography with electrochemical detection to measure plasma and platelet catecholamines in 24 normal subjects. In the same subjects platelet function was assessed by measuring platelet aggregation in response to adenosine 5'-pyrophosphate, thrombin, adrenaline and collagen. Platelet sensitivity to prostacyclin was also examined. Platelet noradrenaline showed a positive correlation with extent of aggregation induced by 'low-dose' collagen (1 microgram/ml). No correlation was seen at the higher collagen concentration. Platelet noradrenaline content also correlated with sensitivity of platelets to prostacyclin. High platelet noradrenaline concentrations appeared to result in decreased sensitivity to prostacyclin. No other correlations were observed. These data suggest that platelet noradrenaline rather than plasma levels may be involved in modifying platelet function in vivo. Local release of platelet catecholamines may affect the platelet/vessel wall interaction, the primary physiological step in platelet activation.