The effects on pregnancy rate of three different treatments to remove intrauterine fluid were assessed in 1267 mares. The mares were mated and allocated, in strict rotation, to four treatment groups: 1) untreated controls, 2) intrauterine infusion of broad spectrum antibiotics, 3) intravenous injection of oxytocin, 4) intravenous injection of oxytocin followed by intrauterine antibiotics. The pregnancy status of the mares was determined 13 to 15 days and 27 to 30 days after ovulation by transrectal ultrasonography. The pregnancy rate of group 4 (72 per cent) was higher than that of group 2 (64 per cent, P<0.01) or group 3 (63 per cent, P<0.01). The pregnancy rates of groups 2 and 3 were higher than that of group 1 (56 per cent, P<0.01). The treatment with antibiotics and oxytocin appeared to have an additive beneficial effect which suggested two different modes of action of the combination treatment, namely antibacterial activity and fluid drainage. In the untreated mares more fluid accumulated in the uterine lumen after mating, and this was the most likely reason for their lower pregnancy rate.