SummaryMale Wistar rats were isolated either immediately after weaning or in adulthood. Then they were decapitated at the last day of the 1st, 3rd, 6th, 9th, 12th and 15th week of isolation and the levels of NA, DA and 5‐HT on whole brains except cerebellum were assayed simultaneously by fluorimetric procedure.When isolation was started immediately after weaning, isolated rats had significantly higher level of brain NA than that of control grouped rats at the 6th and 9th week of isolation and then the brain NA level was reduced to control level, on the other hand when isolation was started in adulthood, brain NA level did not change significantly between the two groups. Brain DA level in isolated rats increased significantly both at the 9th week in post‐weaning rats and at the 12th week in young‐adult rats. Brain 5‐HT level did not change significantly by isolation either immediately after weaning nor in adulthood. As for regional distribution of monoamine levels, rats isolated for nine weeks at weaning had significantly increased NA and DA levels in the situs that the cell bodies of noradrenergic and dopaminergic neurons were located.Namely, it was found that different effect of isolation on the level of brain NA occurred by the difference of age when social isolation had been started, and these data were confirmed by repeate