30 inpatients suffering from major depression (DSM-III-R), who did not fulfill the criteria of seasonal affective disorder (SAD), were treated with either doxepin or amitripytyline as monotherapy and supportively with bright light for 14 days. From days 15 to 19, bright light was replaced by dim light. 18 drug-free control subjects underwent an analogous sequence of bright- and dim-light applications. Phototherapy was applied between 06:00 and 07:30. Heart-rate (HR) analysis was performed in the patients and control subjects before and after the 5th session of bright and dim lights, respectively. 12 patients (40%) experienced improvement of mood during bright-light therapy (group I) while 18 (60%) did not (group II). Patients of group I, who reached significantly higher scores in the seasonal pattern assessment questionnaire than patients of group II, showed an increase of the coefficient of HR variation (HRV) during deep breathing as well as an increment of the high-frequency (HF) peak of spectral analysis exclusively after the bright-light sessions. Patients of group II did not show a significant alteration of these parameters, neither under the conditions of bright-light treatment nor under dim light. The control subjects experienced an increment of the HF power exclusively after bright light. The results suggest that a distinct subgroup of patients with non-SAD major depression shows a more pronounced light-associated increment of parasympathetically controlled cardiac functions than the other depressed patients and the controls.(ABSTRACT TRUNCATED AT 250 WORDS)