Changes in tyrosine hydroxylase (TH) activity in the whole hypothalamus and discrete nuclear regions of the limbic system were observed to occur from 10 to 40 days of age in the female rat. In the whole hypothalamus, TH activity increased between 10 and 30 days of age then decreased at 40 days of age. In the preoptic (POA) and suprachiasmatic nuclear areas (SCN), TH activity increased to peak values at 20 days then declined to baseline values at 25 days. However, an increase in TH activity of SCN was again noted at 39 days of age. TH activity in the amygdala area (AMYG) increased to peak values at 25 and 30 days with a return to baseline levels at 35 days, while in the median eminence (ME), TH activity increased at 20 days and remained elevated through 39 days of age. These results show that temporally distinct changes in TH activity occur during development in various areas of the brain. These changes may represent maturation of neuronal control systems known to be involved in adult reproductive function.