Vitamin B-6 100 mg given daily throughout the menstrual cycle was compared with placebo in a randomized, double-blind crossover trial in 34 women who suffered from premenstrual tension. Vitamin B-6 was no better than placebo. There was a substantial period effect, as the women evidenced a considerable preference for the second drug they received, irrespective of whether this was vitamin B-6 or placebo. Blood magnesium was measured; no significant difference was found between the 34 women with premenstrual tension and 10 healthy women without such complaints. Vitamin B-6 caused a small but statistically significant rise in blood magnesium level. In the individual patients, no correlation was found between changes in blood magnesium and premenstrual symptoms.