The acute effects of low oral doses of Δ9-tetrahydrocannabinol (THC), cannabinol (CBN) and cannabidiol (CBD) administered alone or in combinations on LH and prolactin (PRL) secretion and on hypothalamic norepinephrine (NE), dopamine (DA) and serotonin (5-HI) dynamics were examined in adult male rats. Plasma LH levels were significantly reduced 60 min after administration of 0.5 mgTHC/kg body weight and 30, 60 and 120 min after administration of THC + CBN or THC + CBD. There were no changes in plasma PRL in response to cannabinoid treatments. The turnover of NE in both the median eminence (ME) and medial basal hypothalamus (MBH) was dramatically affected by all the cannabinoid treatments. Complete suppression of NE turnover occurred 30 min post-THC and 120 min post-THC + CBN in the ME and 120 min post-THC + CBD in the MBH. Cannabinoids did not significantly affect DA turnover in the MBH or the content of NE, DA, 5-HT or 5-hydroxyindole-3-acetic acid in either the ME or MBH. These data demonstrate that treatment of adult male rats with a low dose of THC suppresses LH secretion and that CBN and CBD potentiate this action of THC. Although the mechanisms responsible for the inhibition of LH release by cannabinoids cannot be positively identified from these experiments, the results suggest that alterations in hypothalamic noradrenergic activity may be involved in this effect