Fifteen inpatients (nine women, six men) aged 50-86 years with DSM-IIIR major depression were treated with electroconvulsive therapy (ECT). Electrode placement (unilateral versus bilateral) and total number of treatments were determined by the patients' own psychiatrists according to clinical indications. Prolactin (PRL) was determined after the 1st, 6th, 7th, 9th, 11th, and final ECT treatments. Subjects were rated with the Hamilton Depression Rating Scale (HDRS) at baseline, after the sixth ECT treatment, and upon completion of ECT. PRL response to unilateral ECT was consistent across treatment for each subject. Percentage PRL increase was significantly higher for bilateral than unilateral ECT (alpha = 0.05). Subjects with final HDRS of < 12 tended to have greater peak, increase, and percentage increase PRL at the first unilateral treatment than subjects with final HDRS of at least 12; these trends approached statistical significance (Kruskal-Wallis one-way analysis of variance; peak: p = 0.059, chi = 3.556, df = 1; increase: p = 0.099, chi = 2.722, df = 1; percentage increase: p = 0.099, chi = 2.722, df = 1). Decrease in HDRS after the sixth treatment failed to show a statistically significant relationship to any PRL parameter at the initial unilateral treatment. Further studies are needed to characterize the relationship between PRL response, seizure activity, and stimulus dosing.