Strips (2.5 x 3.5 cm) of myometrium alone (MYO) or endometrium/myometrium (ENDO/MYO) were removed from the pregnant horn of sheep (Day 110 of gestation) and transplanted to sites within the omental fat. These explants developed regular bursts of electromyographic (EMG) activity over a period of 7-10 days, as well as a dose-dependent stimulatory response to oxytocin (50-200 mU i.v.). The frequency (per 2 h) of EMG bursts in the MYO (5.3 +/- 0.2) and ENDO/MYO (5.2 +/- 0.3) explants was significantly greater (P less than 0.05) than that of the uterine myometrium (3.0 +/- 0.1), while burst duration (min) in MYO (4.1 +/- 0.2) and ENDO/MYO (4.1 +/- 0.2) explants was significantly (P less than 0.05) less than in the uterine myometrium (7.3 +/- 0.1). The EMG bursts were asynchronous between the explants and uterus, although systemic administration of oxytocin produced a synchronous burst of EMG activity in all three tissues. No differences in EMG activity or responsiveness were apparent between MYO and ENDO/MYO explants. Histological examination of the explant tissue revealed the presence of smooth muscle fibres regularly orientated into two layers; some loss of endometrial tissue was apparent in ENDO/MYO explants. To validate the mechanical integrity of this model we examined the in-vitro contractile activity of myometrial strips prepared from the explants. The strips developed regular spontaneous contractions and demonstrated a dose-dependent stimulation in response to the addition of oxytocin (10(-10) to 10(-4) M) to the bath fluid. These results suggest that spontaneous contractures during pregnancy are probably not due to pulsatile release of stimulants into the systemic circulation, or the direct diffusion of stimulants from intrauterine tissues to the myometrium but are probably caused by factors within the myometrium itself.