Cholinergic antagonists, such as scopolamine, atropine, and hemicholinium-3, have been found previously to inhibit lordosis in ovariectomized rats primed with estrogen and progesterone. The present study further examined this effect using intact cycling female rats. Cycling was determined by daily monitoring of sexual behavior and vaginal cytology. In the first experiment, intraventricular administration of the muscarinic receptor blocker, scopolamine, was found to significantly inhibit lordosis behavior during natural estrus (10 or 20 micrograms bilaterally). In the second experiment, systemic administration of scopolamine was also found to significantly inhibit lordosis behavior during natural estrus (4 mg/kg, IP). Administration of the cholinergic antagonist did not significantly interrupt cyclicity patterns. These results indicate that central cholinergic muscarinic systems contribute to the regulation of lordosis during natural behavioral estrus in intact female rats.