P300 event related potential (ERP) was recorded using the auditory 'oddball' paradigm in 17 melancholic, drug-free depressive patients and 22 normal controls. Repeat ERP recordings were obtained from 13 patients after recovery with ECT. Pretreatment P300 latencies were not different between patients and controls, nor did they change in the depressives following recovery. Pretreatment P300 amplitudes in depressives were smaller (P < 0.02) than in controls and negatively correlated with severity (P < 0.05). P300 amplitudes significantly increased in patients following recovery (P < 0.05) and normalized. P300 may be a state marker for melancholic depression.