We have recently reported altered levels of protein kinase A and Rap1 in patients with bipolar disorder. The purpose of the current investigation was to assess the levels of these proteins in platelets from untreated euthymic and depressed patients with major unipolar depression. Platelets were collected from 45 drug-free unipolar patients (13 euthymic and 32 depressed) and 45 healthy subjects. The levels of protein kinase A and Rap1 were assessed by Western blot analysis, immunostaining and computer-assisted imaging. The immunolabeling of the regulatory subunit type II of protein kinase A and that of Rap1 was significantly lower in untreated depressed patients compared with untreated euthymic patients and healthy subjects. No significant differences were found in the immunolabeling of both the regulatory type I and the catalytic subunits of protein kinase A among groups. Levels of the regulatory subunit type II of protein kinase A and Rap1 are altered in platelets of unipolar depressive patients. These findings may provide new insight about the relationship between components of cAMP signaling and affective disorders.