Levels of serotonin (5-HT) and metabolite, 5-hydroxyindole-acetic acid (5-HIAA), dopamine and metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), and norepinephrine (NE) were measured in microdissected brain areas at 14.30 and 19.30 h on diestrus and proestrus in rats. Lordosis, sexual behavior, was exhibited only by proestrous females at 19.30 h. The content of monoamines and/or metabolites changed from afternoon to early evening of diestrus in a number of brain regions. On proestrus, during the time when females became sexually receptive, additional changes appeared and many diestrous changes were reversed or amplified. Proestrus-specific changes were found in areas provinding the descending circuit for regulation of lordosis, the medial preoptic nucleus (mPOA), ventromedial nucleus of the hypothalamus (VMN) and midbrain central gray (MCG), and in areas outside of endocrine control centers such as parietal cortex (Ctx) and dorsal raphe nucleus (DR). Thus, these data are consistent with previous studies showing functional changes in monoaminergic transmitters during the day/night cycle and during hormonal induction of neuroendocrine events. NE levels increased between 14.30 and 19.30 h on proestrus in most brain areas, ranging from 20% increases in mPOA and Ctx to a 220% increase in the DR. NE levels decreased on proestrus in the VMN. The direction of proestrous changes in NE were reversed in some, but not all of the areas, between the afternoon and evening of diestrus. Dopamine was detectable in all areas sampled, and the metabolites HVA and DOPAC were detectable in some but not all areas. In the mPOA, DOPAC increased 100% during proestrus but was unchanged during the same period on diestrus suggesting a facilitatory effect of dopamine on lordosis in this area. Dopamine terminals in the MCG may inhibit behavior since HVA increased 150% between 14.30 and 19.30 h on diestrus and decreased 40% during the same interval on proestrus. For the serotonergic system, 5-HIAA levels increased 45% in the mPOA and 200% in the MCG from the afternoon to evening of proestrus but did not change on diestrus. In the VMN, 5-HIAA increased 60% between 14.30 and 19.30 h on diestrus but did not change during the same time on proestrus. 5-HIAA also increased 580% in the DR nucleus and 70% in the Ctx during proestrus but not diestrus. These data are consistent with previous studies suggesting that 5-HT terminals regulate lordosis responding on proestrus through hormone-dependent modulations at both facilitatory and inhibitory sites. In the VMN, where 5-HT inhibits lordosis, gonadal hormones appear to decrease the availability of 5-HT, and in the mPOA and MCG, where 5-HT may facilitate lordosis, gonadal hormones appear to increase the availability of 5-HT.