Aim of this study was the examination of relationships between age, parity, incidence of endometrial fibrosis, early pregnancy rate and uterine blood flow in mares. Material and methods: Twenty-two mares were examined between days -2 and 12 (0 = ovulation). Animals were divided into two groups according to age and parity. Mares with an age ranging from 3 to 9 years were characterized as "young" (n = 11 ) and animals with an age of 10 to 19years as "old". Ten of the mares were nulliparous and 12 uni- or pluriparous. Endometrial fibrosis was determined according to criteria by Kenney and Doig (1986). Results: Out of the 22 mares, 13 showed an endometrial fibrosis (category Ma, lib and III, respectively). After insemination with extended semen 8 of the mares became pregnant. The investigations were performed with a colour Doppler sonograph equipped with a 7.0 MHz microconvex-probe. Blood flow was visualized in both uterine arteries. Time-averaged maximum blood flow velocity (TAMV), and vascular-diameter (D) were determined and blood flow volume (VOL) was calculated. VOL values showed similar cyclic changes in all mares (p < 0.05). Cyclic changes in VOL were caused by variations in TAMV; D did not change during the examination period (p > 0.05). VOL was higher (p < 0.05) in aged, uni- or pluriparous and mares with endometrial fibrosis than in "young", nulliparous and mares without endometrial fibrosis. In contrast to pregnant animals uterine VOL values increased (p < 0.05) in barren mares within 12 hours after insemination. Conclusion and clinical relevance: The results show that the investigation of uterine blood flow by transrectal colour Doppler sonography provides important information concerning the fertility of mares. © 2008 Schattauer GmbH.