The administration of platinum-based chemotherapy induces serotonin release from the enterochromaffin cells, causing nausea and vomiting. This study was conducted to evaluate parameters of serotonin metabolism following platinum-based chemotherapy given in combination with the serotonin type-3 antagonist tropisetron as an antiemetic agent. In nine chemotherapy-naive patients with disseminated germ-cell tumors, parameters of serotonin metabolism in both blood and urine were evaluated during two consecutive courses of platinum-based chemotherapy. Serotonin concentrations in platelet-rich plasma and platelet-poor plasma as well as urinary 5-hydroxyindoleacetic acid (5-HIAA) and serotonin levels were measured during the full length of the courses. By means of comparison with the antiemetic agent chlorpromazine, used on day 1 of the first course only, the effect of the serotonin type-3 antagonist tropisetron, the antiemetic agent used during the rest of the courses, on these parameters was studied. Clinical effects were also recorded. No change in the parameters of serotonin metabolism could be demonstrated during either course by the serotonin type-3 antagonist tropisetron. Also in vitro, no effect of tropisetron on the active serotonin uptake by platelets was found. Serotonin levels in platelets showed no correlation with emetic response. However, the platelet serotonin content decreased significantly between the first and the second course (P < 0.01). The significant reduction in platelet serotonin content observed between the first and the second course indicates a depletion of total body serotonin. The role of a serotonin type-3 antagonist might be affected by the altered serotonin equilibrium during later courses of chemotherapy.