We have determined the levels of plasma beta-thromboglobulin (beta-TG) and platelet factor 4 (PF4) and the platelet count of 50 manic-depressive patients under lithium treatment and of 30 healthy people. There was no significant difference between the platelet count of the patients and that of the control group. However, when the patient group was classified according to the duration of treatment with lithium, the platelet count of patients treated for less than 1 year (322.7 +/- 80.1 x 10(3)/mm(3)) was significantly higher than that of those treated for 1 to 5 years and more than 5 years (263.2 +/- 71.8 x 10(3)/mm(3) and 212.4 +/- 24.1 x 10(3)/mm(3) respectively). The beta-TG and PF4 levels of the patients were significantly higher than those of the control group but there was no correlation between these levels and the duration of the treatment. beta-TG and PF4 are considered as useful markers of platelet activation. Therefore our findings suggest that lithium treatment affectss the functions of platelets. (C) 1998 Lippincott-Raven Publishers.