In Exp. 1, 4 mares were given oxytocin intravenously (10 i.u./500 kg body wt) daily between Days 9 and 14 (Day 0 = day of ovulation) when pregnant and on Days 9-14, 16, 18, 20 when non-pregnant (not inseminated). In the non-pregnant mares the increase in plasma PGFM response to oxytocin was greater at Day 13 (235 +/- 54 pg/ml) than at Day 11 (113 +/- 38 pg/ml; P less than 0.05) and was maximum at Day 16. However, these animals did not return to oestrus and plasma progesterone did not fall below 4 ng/ml. There was no significant increase in response to oxytocin between Days 9 and 14 in the pregnant animals. In Exp. 2, when these same mares were challenged with oxytocin on alternate day (Days 9, 11 and 13 for pregnant mares, Days 9, 11, 13, 15, 17 and 19 for non-pregnant mares) there was a significant difference in the response between non-pregnant and pregnant mares by Day 13 (383 +/- 19 pg/ml vs 88 +/- 9 pg/ml; P less than 0.005). Plasma progesterone concentrations declined normally and the mares returned to oestrus. During oestrus the response to oxytocin decreased dramatically in mares receiving oxytocin on alternate days, and no response was seen by Day 19. The response also declined after Day 16 in the non-pregnant mares that had daily injections of oxytocin even though plasma progesterone remained elevated. The decreased response coincided with the increase in plasma oestrogen concentrations, suggesting that oestrogens play a role in the control of the response.(ABSTRACT TRUNCATED AT 250 WORDS)