Male Sprague-Dawley rats were pretreated subcutaneously with either progesterone (3 mg/kg per day) in a vehicle or a vehicle only for 3 days. Antral and gastroduodenal junctional tissues (GJT) were excised from both groups of animals and prepared for in vitro mechanical measurements. Responses from the circular muscle axis of these tissues were recorded with strain gauge transducers over a 30-min period. Chemical stimulation of the tissue was achieved with a muscarinic agonist, bethanechol chloride. Log-dose response curves suggested that untreated antral tissue generated stronger contractile activity than untreated GJT on an equal weight basis at bethanechol dose levels of 6.4 X 10(-6) M to 1 X 10(-4) M (P less than 0.005). Antral tissue and GJT contractile activity from the progesterone pretreated animals was significantly reduced (P less than 0.01) compared to the corresponding tissues from untreated animals at bethanechol dose levels of 6.4 X 10(-6) M and 1.28 X 10(-5) M. Progesterone pretreatment appeared to have little effect on the contractile frequency of either tissue. These results suggest possible progesteronic influences on contractile force in gastrointestinal smooth muscle.