To determine the effect of estrogenic and progesteronic activity on lower esophageal sphincter (LES) circular muscle, studies were performed on 20 adult opossums. Does-response curves on circular smooth muscle strips from the LES were constructed for gastrin and acetylcholine alone, and with 17beta-estradiol and/or progesterone added. Each female hormone significantly decreased the maximal LES muscle responses to gastrin and acetylcholine. A combination of 17beta-estradiol and progesterone abolished the response to gastrin. In contrast, the male sex hormone, dihydrotestosterone, had no effect. In conclusion, administration of estrogen and progesteron, but not dihydrotestosterone, in vitro reduced LES muscle responses to gastrin and acetylcholine. These studies suggest that the female sex hormones can alter LES function and potentially may be of importance in the pathogenesis of heartburn of pregnancy.