Within the ventromedial nucleus of the hypothalamus (VMN), serotonin exerts a dual role in the control of female rat lordosis behavior. Most emphasis has been placed on 5-HT1A and 5-HT2 receptors, which inhibit and facilitate the behavior, respectively. In the current experiment, a potential role for VMN 5-HT3 receptors in the control of lordosis behavior was examined. Ovariectomized rats, hormonally primed with 25 microg estradiol benzoate and 500 microg progesterone, received bilateral VMN infusions with 100 ng, 250 ng or 500 ng of the 5-HT3 receptor antagonist, tropisetron. In these rats, there was a dose-dependent decline in both the lordosis to mount (L/M) ratio and in the quality of the lordosis reflex with 500 ng tropisetron producing the most consistent change in lordosis behavior. Relative to hormone-primed, ovariectomized rats, lordosis behavior of proestrous females was less affected by VMN infusions with the 5-HT3 receptor antagonist. The 5-HT3 receptor agonist, m-chlorophenylbiguanide (mCPBG), attenuated the effect of tropisetron; of the three mCPBG doses (500 ng, 1000 ng, 1500 ng) examined, 1000 ng was the most effective, perhaps because, alone, 1500 ng mCPBG slightly reduced lordosis behavior. These observations emphasize the potential role for VMN 5-HT3 receptors in the control of lordosis behavior.