After parturition, Thoroughbred mares were mated at the first post-partum oestrus (N = 24) or at a subsequent oestrus (N = 12). All mares were examined daily for: oestrous detection, palpation per rectum of the genital tract, vaginoscopic examination and cervico-endometrial cytology. Pregnancy diagnosis was carried out at Days 18, 35 and 45 after mating. An identical first service conception rate of 50% was found in both groups. The number of neutrophils followed a descending profile to only scattered cells at the first post-partum oestrus and in Group II mares remained at this very low level during the period of study. The percentage of histiocytes and eosinophils increased on Days 10 and 17, and 5 and 6 post partum, respectively. The percentage of lymphocytes remained low and constant during the period of study. Bacterial flora decreased from Days 2 to 9 and increased from Days 13 to 17 post partum. Cellular necrosis and erythrocytes decreased and ciliocytopholia increased as mares approached the first post-partum oestrus. A positive correlation was found between amount, colour, viscosity and turbidity of secretions and all cellular types, ciliocytopholia, cellular necrosis and bacterial flora. The number of neutrophils was positively correlated with the percentage of eosinophils, bacterial flora and cellular necrosis but had a negative association with the presence of ciliocytopholia. Two mares that did not re-establish cyclic ovarian activity after parturition had delayed uterine involution. Mares not conceiving at the first post-partum oestrus exhibited a more prolonged presence of cellular necrosis and erythrocytes and an increased presence of bacterial flora and lymphocytes as compared to mares conceiving at this period.