To test the hypothesis that the antidepressant effects of bright light in seasonal affective disorder are mediated by the suppression of melatonin, 19 patients with this disorder were given atenolol, which suppresses melatonin secretion, and placebo in a double-blind crossover study. No difference in antidepressant efficacy was found between drug and placebo in the sample as a whole, which argues against the melatonin hypothesis of phototherapy. However, in three of the patients atenolol provided repeated, marked, and sustained relief of symptoms, suggesting that it may be useful in treating the winter depressive symptoms of some patients with seasonal affective disorder.