To reveal neurophysiological correlates of treatment efficacy of late onset depression, EEG spectral power, peak latencies of the "late" components of auditory cognitive evoked potentials, and sensorimotor reaction time have been analyzed in two groups of elderly patients, aged 53-72 years, with prolonged psychogenic depressive reaction (F43.21 by ICD-10) and with endogenous depression (F33.1 and F31.3 by ICD-10) during the treatment with antidepressants. Baseline depression severity has been associated with the EEG signs of the decreased functional state of anterior areas of the left hemisphere, and of the increased activation of the right hemisphere (especially, of its temporal regions). The pronounced improvement of clinical condition of patients after psychopharmacotherapy with antidepressants led to the decrease of peak latencies of the "late" components (Р2, N2 и Р3) of auditory cognitive evoked potentials, and to the acceleration of sensorimotor reaction time that have been associated with the EEG signs of the improvement of the functional state of posterior brain areas, and of the facilitation of inhibitory processes in the right hemisphere (especially, in its frontal, central and temporal regions), and of the more pronounced activation of frontal areas of the left hemisphere. The results are in line with the views on systemic character of brain functioning impairment in depression, as well as on the preferential role of the left hemisphere in the control of positive emotions, and of the right hemisphere role in the control of negative emotions as well as in the pathogenesis of depression.