This is a preliminary report for understanding biochemical mechanism underlying social isolation-induced behavioral changes.Both male and female Wistar rats were isolated since weaning and every two weeks general behavior in the open-field situation was observed and every four weeks muricide was checked by means of putting a mouse in the rat's home cage for 30 minutes.After 12 weeks from the start of isolation, the rats were decapitated and the levels of NA, DA, serotonin and 5-HIAA on whole brains except cerebellum were measured by fluorimetric procedures.Isolated rats showed significantly higher frequency of blocks traversed than those of the grouped rats on the 91st and the 105th day after birth, and brain noradrenaline level of isolated rats was significantly higher than that of grouped, whereas there were no significant changes in the brain levels of DA, serotonin and 5-HIAA.