The platelet serotonin uptake kinetics was studied in 19 women with premenstrual tension syndrome (PMS) and 19 age-matched symptom-free controls during the pre- and postmenstrual phases. Both groups exhibited stable V_max and K_m values at the two phases of the menstrual cycle. There were no differences between the two groups regarding the level of V_max and K_m, respectively. The effect of season (p < 0.001), with lowered values of V_max during spring compared to corresponding values during the later half of the year, was identical in both groups. A tendency to lowered V_max values in the follicular phase in women with PMS during spring may indicate that women with PMS are more vulnerable to developing depressive episodes during this period than symptom-free women. The blood platelet count in whole blood was decreased in the luteal phase in the control group but remained stable in the PMS group. The effect of vitamin B₆ on clinical symptoms and the platelet serotonin uptake during the luteal phase was investigated in a double-blind crossover design. Vitamin B₆ treatment did not significantly improve the PMS symptoms. There was no effect of treatment on K_m. V_max increased significantly in response to treatment (F = 9.6; p < 0.001) in both groups, but there was no significant difference between placebo and vitamin B₆.