MAO activities were studied in 4 months old and 19 months old Wistar rats. MAO-B activity was higher in brain stem and hemispheres of old rats as compared with that of young animals. MAO-A activity was lowered only in the stem and was not changed in the hemispheres of old rats. Actinomycin D increased MAO-A activity in the brain stem of young rats and lowered MAO-B activity in the hemispheres of old rats. Emotional stress increased MAO-A activity in the stem and hemispheres of both young and old animals. Actinomycin D abolished the stress effect on these two forms of MAO both young and old rats. Delta-sleep including peptide at the dose of 120 micrograms/100 g increased MAO-A activity and lowered MAO-B activity in young animals but did not affect the MAO-A activity in old rats.