Severe stress, with increased secretion of adrenaline, is likely to cause platelet activation. The aim of the present study was to investigate if moderate stress, which usually is not accompanied by adrenaline secretion, could induce activation of platelets, as measured by changes in the plasma concentrations of platelet factor 4 (PF-4). Noise stimulation (100 dBA for 10 min) caused a significant increase in the diastolic (10%, p less than 0.01) and mean arterial pressures (4%, p less than 0.01) of 10 healthy male volunteers. The plasma levels of PF-4 and the venous platelet concentrations were, however, unaffected during noise exposure. The results therefore suggest that stress not accompanied by adrenal medullary activation, does not induce platelet activation.