This paper reports the behavioral responses to m-chlorophenylpiperazine (m-CPP), a serotonin agonist, in patients with seasonal affective disorder (SAD) and controls during the summer. Results are compared with the responses of SAD patients and controls given m-CPP in the winter. Results of the winter study were reported earlier by our group. Baseline Hamilton depression ratings in SAD patients were significantly lower in the summer than in winter (p < 0.05). Additionally, in both SAD patients and controls, there were seasonal differences on the National Institute of Mental Health (NIMH) self-rating scale items: "depressed affect," "dysphoria," and "functional deficit" at baseline. The behavioral responses to m-CPP across seasons differentiated patients from normals only in the "activation/euphoria" item, on which a far greater response was seen in patients than in controls during the winter. This behavioral response may be a state marker for winter depression, as it was significantly reduced after light treatment of these patients in the winter, and in the summer. SAD patients responded differently from controls on "altered self-awareness" and "dysphoria" independently of seasons, and these responses may be considered as possible trait markers for this condition. These results provide further evidence of a possible deficiency in serotonergic transmission in seasonal affective disorder.