The recent discovery of peripheral markers of central neurotransmitter systems has broadened the scope of biological research in psychiatry. Human platelets resemble presynaptic serotoninergic neurons and permit us to investigate the involvement of the serotonin system in the pathophysiology of mood disorders. In particular, platelets show an active uptake of serotonin and the related 3H-imipramine (3H-IMI), similar to cerebral binding sites. We evaluated 3H-IMI binding in a group of 30 bipolar patients as compared with healthy controls. In 20 patients, platelet 14C-5HT uptake was also measured. The results showed no difference in IMI binding parameters between bipolar patients and healthy controls. However, the patients showed a lower Vmax of 14C-5HT uptake than the controls. These findings suggest that bipolarity influences one of the main components of the 5HT transporter complex in platelets.