In an attempt to clarify the possible role of a serotonergic and dopaminergic dysfunction in obsessive-compulsive disorder (OCD), we measured platelet 3H-imipramine (3H-IMI) binding, serotonin uptake, and platelet sulfotransferase activity in 17 drug-free OCD patients and an equal number of healthy controls. Serotonin uptake and 3H-IMI binding sites in platelets have been shown to constitute peripheral markers of those present in presynaptic serotonergic neurons. Sulfotransferase, an enzyme involved in the catabolism of phenolic compounds and of cathecholamines such as dopamine, has similar kinetic characteristics in brain and platelets. Our results showed a lower number of 3H-IMI binding sites and a higher level of sulfotransferase activity in OCD patients compared with those in controls. These preliminary results suggest involvement of both the serotonin and dopamine systems in OCD.