Control of the HPG axis involves a rapid (30 min) inhibition of LH (GnRH) release by E2. The time course of this effect is faster than expected for a purely transcriptional mechanism of E2 action. To elucidate the mechanism of E2 action, intracellular recordings in TTX were performed in guinea pig hypothalamic GnRH neurons. These neurons were directly hyperpolarized by both the mu-opioid agonist, DAMGO (Tyr-D-Ala-Gly-MePhe-Gly-ol, 9 mV) and the GABAB agonist, baclofen (18 mV) by opening K+ channels. Schild analysis with naloxone (Ke = 2.4 nM) confirmed that mu-opioid receptors mediated the effect of DAMGO. E2 also directly hyperpolarized GnRH neurons by opening K+ channels. Coupled with previous work showing a rapid effect of E2 to alter mu-opioid potency (1), a model is presented in which E2 rapidly inhibits GnRH neurons through parallel, possibly synergistic pathways.