Uterine electromyography was performed by chronically implanted surface electrodes in 3 pony mares during spontaneous estrus cycles and following luteolysis induced by a prostaglandin analogue (fluprostenol). Distinct patterns (3) were recognized during the estrus cycle: during estrus well defined phases of activity with closely grouped high-amplitude spikes were separated by long periods (10-45 min) of complete inactivity; during diestrus more diffuse phases of activity with low-amplitude spikes were separated by variable periods of relative inactivity and during luteolysis, short and frequently occurring phases of activity were propagated between the 2 electrodes on 1 uterine horn. A similar pattern also occurred between 1 and 3 h after injection of fluprostenol. Peripheral plasma progesterone, but not total unconjugated estrogen, concentrations were closely related to characteristics of the myographic activity during the cycle. Insemination during estrus and injection of fluprostenol during diestrus caused a marked and prolonged increase in myometrial electrical activity. Almost any non-specific environmental stimulus, including entry to the mare''s stall and the sound of human voices, was able to influence myometrial activity at any stage of the cycle. Similar changes in activity were induced by palpation of the genital tract per rectum and vaginoscopic examination, but these were of brief duration. The normal resting pattern of activity was quickly re-established after completion of the manipulations.