1 Plasma levels and platelet 5-HT uptake characteristics were determined at baseline and at various times during a long-term study comparing fluvoxamine and lithium prophylaxis. 2 In the fluvoxamine-treated patients after 12 and 24 weeks of treatment, Km was significantly increased and y, the mean overall uptake, significantly decreased compared to the patients' own baseline values and to lithium treatment. No significant change was noted for Vmax. 3 For fluvoxamine, significant negative relationships were established between the Vmax of platelet 5-HT uptake and plasma levels. 4 The trial had to be discontinued prematurely for some patients due to apparent lack of efficacy, unwanted effects or a combination of both in the fluvoxamine-treated patients. Low plasma levels of fluvoxamine may have contributed to the apparent lack of prophylactic efficacy of the drug. 5 Reference is made to the activity of receptor systems in patients receiving lithium and the consequent changes occurring after the administration of a potent 5-HT re-uptake inhibitor.