1. We have used high-performance liquid chromatography with electrochemical detection to measure content of adrenaline and noradrenaline in platelets in 13 normal subjects at rest. 2. Subjects were exercised to raise plasma catecholamine levels and promote the platelet release reaction. 3. There was a significant positive correlation between plasma noradrenaline concentrations and platelet noradrenaline content. 4. Platelet/plasma concentration ratios were 1855 for noradrenaline and 268 for adrenaline at rest and 473 and 152 respectively after exercise. 5. Plasma noradrenaline levels positively correlated with age. 6. Determination of platelet factors released to the plasma showed increases of β-thromboglobulin and platelet factor 4 with exercise, whereas thromboxane B 2 remained unchanged. No change in platelet catecholamine levels occurred with exercise and no correlations were observed between platelet catecholamines and released platelet factors. 7. These data suggest that plasma catecholamine levels influence platelet content and that noradrenaline and adrenaline are concentrated in platelets.