Collagen (5-160 μg/ml) induced release of free and sulphate conjugated serotonin (5-HT) from platelets in platelet-rich plasma (PRP) was examined in normal human subjects. Collagen stimulated release of free 5-HT to the plasma (platelet-poor; PPP) increased in a dose dependent manner (P < 0.0001) and was mirrored by a decline in platelet free 5-HT content (P < 0.0001). The half-maximum response occurred at a collagen concentration of 10.5 μg/ml. Non-specific (resting) release of free 5-HT represented 2.5% of the total free 5-HT (sum of PPP and platelet concentrations). On stimulation with 5,10,20,40,80 and 160 μg/ml collagen, PPP free 5-HT concentrations were 24.2, 45.5, 63.2, 74.9, 82 and 89.7% of the total respectively. Collagen also elicited dose dependent release of sulphate conjugated 5-HT to PPP (P < 0.05), which was reflected by a corresponding decline in platelet sulphate conjugated 5-HT content (P < 0.0001). Half maximal release of sulphate conjugated 5-HT occurred with 6 μg/ml collagen. Under resting conditions platelet and PPP sulphate conjugated concentrations were respectively 6 and 35.1% of total (sum of free and sulphate conjugated) 5-HT concentrations, whilst under collagen (160 μg/ml) stimulated conditions these concentrations represented 12.8 and 7.7% of the total. Mobilisation (i.e. hydrolysis) of platelet sulphate conjugated 5-HT did not occur on collagen stimulation, as evidenced by the absence of alterations in total sulphate conjugated 5-HT concentrations (sum of PPP and platelet sulphate conjugated 5-HT concentrations). PPP concentrations of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HIAA) were unaltered by collagen stimulation indicating that it did not influence 5-HT metabolism. It is concluded that release of platelet sulphate conjugated 5-HT, in addition to free 5-HT, occurs on platelet activation by collagen, although the sulphate conjugated fraction represents only a minor proportion of the total (free and sulphate conjugated) 5-HT released. Further studies will establish the physiological significance of platelet sulphate conjugated 5-HT.