Changes in hypothalamic concentration and turnover index (TI) of norepinephrine (NE), dopamine (DA), and serotonin (5HT) were studied during the first estrous cycle at the onset of puberty. Rats were killed when 40% of the population showed open vaginas. They were classified according to the state of their reproductive tract and serum LH values, as in anestrus, early proestrus, late proestrus, estrus, or diestrus. Serum LH, PRL, and hypothalamic LHRH were measured by RIA. The TIs for DA and NE were measured by estimating their rate of decrease after administration of alpha-methyl-para-tyrosine, and for 5HT-TI by its increase after pargyline administration. An increase in NE-TI in the hypothalamus was observed between anestrus and early proestrus, a decrease in DA-TI during early and late proestrus, and a small increase in 5HT-TI between early and late proestrus. LH and PRL peaks were observed at late proestrus when DA-TI and NE-TI reached their lowest values and hypothalamic LHRH concentration was failing. After late proestrus, the DA-TI rose sharply, NE-TI remained essentially unchanged. 5HT-TI fell, and serum LH and PRL declined to basal levels. These results suggest that the LH and PRL surges at late proestrus during the first estrous cycle at puberty are associated with an increase in NE-TI at early proestrus, a progressive decline in DA-TI at early and late proestrus, and an increase in 5HT-TI at late proestrus.