Summary The percentage of Technetium 99m‐albumin colloid (99mTc‐μAA), a radiocolloid, cleared from the uterine lumen within 4 h of intrauterine infusion, was measured in 15 mares during 2 consecutive cycles, on Day 3 of oestrus and 48 h after ovulation. Four nulliparous (Group 1) and 4 multiparous (Group 2) mares were classified as resistant and the remaining 7 multiparous mares were classified as susceptible (Group 3) to endometritis. Mares in Groups 1 and 2 cleared more 99mTc‐μAA from their uteri than did mares in Group 3 during oestrus (P<0.01) and 48 h after ovulation (P<0.001). In the Group 1 + 2 mares, >50% of the colloid was cleared in 7 and none in the remaining mare, apparently related to lack of cervical relaxation. Mean percentage of 99mTc‐μAA cleared by Group 3 mares was negligible (<5%), but some 99mTc‐μAA was cleared by 3 of the 7 mares during 4 of the 6 studies conducted. Clearance of radiocolloid infused into the uterus of 3 reproductively normal mares during dioestrus was negligible. 99mTc‐μAA infused into the uterus did not adversely affect endometrial integrity as determined by endometrial biopsy. Mares tolerated the procedures well. We conclude that scintigraphy can be used to detect impaired mechanical clearance of the uterus: reproductively normal mares clear >50%99mTc‐μAA within 2 h of infusion whereas those susceptible to endometritis or mares with poor cervical dilatation may exhibit delayed uterine clearance.

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Summary The effects of oxytocin on the percentage of technetium 99m albumin colloid (99mTc‐μAA), cleared from the uterine lumen was measured in 13 mares. Scintigraphy was performed during 4 consecutive oestrous cycles, on Day 3 of oestrus during Cycles one and 3 and 48 h after ovulation during Cycles 2 and 4. Oxytocin (20 iu) was given i.v. after the initial scintigraphy image during Cycles 3 and 4. Seven multiparous mares (Group 1) were classified as ‘susceptible’ and 6 mares (2 nulliparous and 4 multiparous; Group 2) were classified as ‘resistant’ to endometritis. All mares cleared >90% of 99mTc‐μAA within 30 min of oxytocin injection. When no drug was given, Group 1 mares cleared negligible amounts of radiocolloid (<5%) and Group 2 mares cleared 50–80%. No mares showed signs of colic after oxytocin administration. It is concluded that oxytocin enhances uterine clearance of radiocolloid and may be useful for treating mares exhibiting impaired uterine clearance.