To seek a possible role of estrogen and progesterone in the development of changes in esophageal function during pregnancy, we produced a state of pseudopregnancy by administration of hormones to a suitable animal model. Twenty-two female opossums weighing an average of 2.5 kg, were divided into two groups. The treated group received intramuscular injections of 100 microgram of estradiol valerate daily from day 1 to day 12 and 15 mg of medroxyprogesterone acetate from day 7 to day 12. The control group received no injections. Both groups underwent manometry on days 1, 7, and 12. Lower esophageal sphincter pressure decreased (P less than 0.05) in treated animals from 58 +/- 13 mm Hg and 57 +/- 11 on days 1 and 7, respectively, to 44 +/- 10 mm Hg on day 12. The lower esophageal sphincter pressure remained unchanged in control animals. In both groups, there was no change in peristaltic wave pressure, duration, or velocity in the distal 6 cm of the esophagus. No abnormal peristaltic phenomena were observed. Esophageal muscle strips prepared from treated animals showed responses to electrical field stimulation of intrinsic nerves that were like those from control animals. The same was true for responses to acetylcholine and pentagastrin. Total tissue water and total tissue potassium content did not differ in treatment and control animals.