Statistically significant diurnal variations in plasma growth hormone (GH) were found to occur in handled male rats. Peak GH values (at midday) appeared to be inversely correlated with the diurnal peak of plasma corticosterone (CS) which occurred after the onset of darkness. Brain amines were examined in the following regions: cortex, striatum, septum, amygdala, pons, midbrain, and hypothalamus. Statistically significant diurnal cycles of serotonin (5-HT) concentration were found in the amygdala and midbrain, while an ultradian cycle of 5-HT was present in the striatum. Striatum and pons also showed significant diurnal alterations in norepinephrine (NE) levels. Ultradian dopamine (DA) cycles were significant in all regions examined. Plasma GH changes were found to be directly correlated with midbrain and amygdala 5-HT levels and with DA levels of all areas except the amygdala. Plasma CS was found to be inversely correlated with striatal and cortical DA and with 5-HT levels of amygdala.