Diurnal variations in motor activity and regional monoamine levels of the brain in both young adult and aged wistar male rats were compared. The young adult group (Y group) consisted of 16 week old rats. The aged group (A group) comprised 19 month old rats. All the animals were housed in a room with an alternating 12 hour dark-light cycle. The temperature was kept at 24℃ and the moisture at 55 % . Motor activity was measured in selected rats by Animex. Both groups were then subdivided into six smaller groups. The animals of these groups were sacrificed by decapitation every 4 hours of the 24 hour period (2, 6 and 10 hours in the dark cycle, and 2, 6 and 10 hours in the light cycle; D2, D6, D10, L2, L6 and L10). Immediately after decapitation, brains were collected, dissected into six different regions (amygdala [Amy], corpus striatum [St], cerebral cortex [Cor] , hippocampus [Hip], diencephalon [Die] and brain stem [B. S.]), and the levels of dopamin [DA], norepinephrine [NE] and serotonin [5-HT] in each determined fluorometrically. The results were as follows. 1. Diurnal variation of motor activity. Both Y and A groups showed a marked difference between the dark and light cycles. Activity increased during the first 4 hours of the dark cycle, then decreased gradually during the rest of the dark cycle. Motor activity decreased markedly immediately after beginning of the light cycle, and remained decreased throughout the light period. The motor activity of the both groups increased transiently a little before the light cycle. Y group activity increased directly before the dark time. A group activity was significantly less than that of Y group, except for 2 time points during the light cycle. A group motor activity was about 50 % lower than that of the Y group. 2. Diurnal variation of DA. Amy from both groups showed a higher levels in the dark time than in the light. St showed two peaks in both groups but the range of variation was wider in A group than in Y group. Cor from both groups showed a higher level in the light time than in the dark. The levels of Hip, Die and B. S. in both groups showed little variation, the range being much smaller in A group. There was no significant difference in the levels of Amy, Cor and Hip in either group, but St, Die and B. S. were significantly lower in A group throughout almost the entire 24 hour period. 3. Diurnal variation of NE levels. Marked variations in Amy, Cor, Hip and Die occurred in both groups. The NE level in the 4 brain regions tended to be lower during the dark time than during the light. The diurnal variation of amine levels in St was smaller in the groups than in the other five regions. B. S. in the Y group showed lower levels during the dark time; however, B. S. from A group showed almost constant levels during the 24 hour period. The NE levels of each groups showed little difference in brain regions other than B. S.. In A group, B. S. showed 1.8 to 9 times higher levels than Y group at all examined time points. 4. Diurnal variation in 5-HT levels. Both groups showed higher amine levels in Amy, Hip and Die during the dark period than in the light. St in Y group showed two peaks at D10 and L6 but the same region in A group showed almost constant amine levels during the diurnal period except for D6. The level in Cor from both groups showed little variation, the range being much smaller in A group than in Y group. The level in B.S. of Y group showed a marked dark-light difference with a high peak at L2 but the same region from A group showed a gradual variation in the amine level with a peak between D10 and L2. Comparison of the 5-HT levels in Y and A groups showed that there was no significant difference in Amy, Hip and Die. The level in St remained high throughout the diurnal period in Y group. The level in Cor showed a higher value during the 24 h period in A group. The amine level in B. S. was higher in the A group at two time points of the dark period than in Y group. The data indicate a characteristic influence of aging on diurnal variations of motor activity and three monoamine contents of the brain. Some regions of the brain showed no difference in DA levels of Y and A groups, whereas other regions showed significantly lower levels in A group. Some regions showed no difference in NE levels of both groups and the other showed significantly higher levels in A group. These results suggest that the turnover rate of DA in some brain regions is accelerated in A group compared to Y group. A group tended to show less diurnal variation in the three monoamine levels of the brain than Y group when related to the reduced motor activity of A group, this suggests that metabolism of these monoamines in related to behavior.