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 Vmax and Km values at the two phases of the menstrual cycle. There were no differences between the two groups regarding the level of Vmax and Km, respectively. The effect of season (p less than 0.001), with lowered values of Vmax during spring compared to corresponding values during the later half of the year, was identical in both groups. A tendency to lowered Vmax 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 Km. Vmax increased significantly in response to treatment (F = 9.6; p less than 0.001) in both groups, but there was no significant difference between placebo and vitamin B6.