We studied the effect of lithium (L), carbamazepine (CBZ), valproic acid (VA), verapamil (VP) and nifedipine (NF) on the specific binding of 3H-imipramine (3H-IMI) to platelet membranes, as compared with clomipramine (CLO). The results showed that VP, NF and CLSO exerted a concentration-dependent inhibition on the IMI binding; CLO had the most potent inhibitory effect. No effect was observed with L. CBZ or VA. These results suggest that while VP and NF interact with the 5-HT transporter complex. L, CBZ and VA do not. Therefore, their effective (or suggested) mood-stabilizing effects are likely to be related to different mechanisms.