Push-pull cannulae were implanted into the arcuate nucleus of pregnant or ovariectomized (OVX) rats, and the perfusate samples were analyzed for biogenic amines and their metabolites. Injection of pargyline, a monoamine oxidase inhibitor, resulted in a decrease in 3,4-dihydroxyphenylacetic acid (DOPAC) and 5-hydroxyindoleacetic acid (5-HIAA) levels in the samples, and detectable amounts of dopamine (DA) and serotonin (5-HT), which were usually not measurable prior to injection. Administration of 5-hydroxytryptophan resulted in a sharp increase in 5-HIAA and 5-HT levels in the perfusates, and no change in DOPAC levels. Push-pull perfusion was done between midnight and 06.00 h on day 8 and 16 of pregnancy. In those rats which showed a nocturnal prolactin (PRL) surge on day 8, 5-HIAA levels were very high compared to those that did not, or compared to those on day 16, which had chronic low PRL levels. DOPAC levels were not significantly different in the 3 groups. Perfusion of medial basal hypothalamic (MBH) fragments taken from mother rats on day 8 of pregnancy during the PRL surge spontaneously released more DA or 5-HT than did fragments taken on day 16 at the same time of day. These results suggest that serotonergic activity in the MBH is higher on day 8 of pregnancy, in parallel with the occurrence of PRL surges, than on day 16 when no surges are present. Dopaminergic activity, as measured by DOPAC levels in push-pull samples, does not appear to be different between the two days.