Abnormalities of platelet serotonin (5-HT) transporter, which are supposed to reflect similar dysfunctions in the central nervous system (CNS), have been reported in obsessive-compulsive disorder (OCD). Other platelet parameters altered in OCD are represented by phenolsulfotransferase (PST) activity, an enzyme involved in the catabolism of catecholic neuro-transmitters, and peripheral benzodiazepine receptors. Since no information is available on the behavior of these putative markers during antiobsessive treatments, the aim of the present study was to measure and compare 3H-imipramine (3H-IMI) binding, which labels the 5-HT transporter, PST activity, and 3H-PK 11,195 binding, which labels peripheral benzodiazepine receptors, in a group of 18 patients with obsessive-compulsive disorder (OCD) before and after a treatment with fluvoxamine versus clomipramine. The results showed that at baseline the patients had a decreased number of 3H-IMI binding sites, which correlated negatively with the Y-BOCS total score, an increased PST activity and no difference in 3H-PK 11,195 binding, as compared with healthy volunteers. After eight weeks of treatment with either clomipramine or fluvoxamine, which was effective in all patients, the number of 3H-IMI binding sites increased significantly toward normal values, while the PST showed no change. These findings suggest that the reduction in 3H-IMI binding sites in OCD may be related to the severity of the illness and possibly to a positive response to serotonin re-uptake inhibitors, and might be considered as a state-dependent marker, whereas the PST activity would seem to be a trait of the illness.