Platelet [3H]-paroxetine binding was analysed in 26 patients (13F, 13M) with obsessive compulsive disorder and 26 normal controls (13F, 13M). For patients with obsessive compulsive disorder, Bmax was 2127 ± 480 fmol/mg protein (mean±SD) compared to control Bmax values of 1926 ± 696 fmol/mg protein. The mean Kd value for the patients was 0.075 ± 0.025 nM and for the controls was 0.076 ± 0.032 nM. Analysis of covariance indicated a significant effect of sex on both Kd and Bmax but no effect of diagnosis (obsessive compulsive disorder versus normal controls) or season of sampling. The data provide no evidence for an abnormality of the platelet uptake mechanism as assessed by the measurement of [3H]-paroxetine binding to the platelet transporter in obsessive compulsive disorder.