Reduced fertility associated with fluid accumulation has been recognized for many years in broodmares. Fluid present in the uterus after ovulation is often associated with mare susceptibility to endometritis. Objective of this study was to determine amount of intrauterine fluid secretion and endometrial oedema in mares at the time of artificial insemination (AI), 6 h, 25 h, and 48 h after AI and at the time of ovulation. A total of 22 cycling light breed mares from MTT Agrifood Research, Ypaja, Finland, were used in the study. Relaxation of the cervix, the number and size of follicles, corpus luteum, and the degree of endometrial oedema were examined by transrectal palpation and ultrasonography every other day. According to the closing and opening time of the cervix which was regulated using Bivona catheter, mares were distributed into 3groups. Group A (n=7): immediately after artificial insemination (AI) the catheter was inserted into the uterus. After 25 h, the catheter was opened and the fluid drained. Group B (n=8): immediately after AI, Bivona catheter was inserted into the uterus. After 6 h and 25 h the catheter was opened and the fluid drained. Group C (n=7)-control group: no catheter was inserted into the uterus. According to our investigation, we concluded that the amount of intrauterine fluid accumulation (IUFA) in mares increased within 25 h after AI. Mares with delayed uterine clearance had a significantly larger amount of intrauterine fluid than mares with normal uterine clearance. Open cervix during oestrus period is important for adequate clearance of intrauterine fluid in mares. Opening of intrauterine catheter at 6 h post AI didn’t diminish intrauterine fluid accumulation at 25 h post AI.