Summary: To achieve the desired reproduction rate of one foal per year per mare and to optimize the reproductive potential of mares that are embryo donors, foal heat should be utilized as a tool for improvement of fertility. The mare is the only domestic animal showing ovulatory estrus shortly after foaling. The uterine involution and the ovarian activity (post partum follicular dynamic) in the early puerperium go through brief periods of time. Fetal expulsion at physiological delivery occurs within a short time and the placenta is expelled few hours later. Such events in the intra-and peri-partal period may be also related to the type of histological tissue that composes the cervix in these animals. The mare's cervix (muscular type) has a fantastic speed related to uterine involution, unlike the other species in domestic animals (cervix fibro-cartilaginous type) (more resistant requesting more time to involute). Nonetheless, fertility during foal heat is reported to be by some authors better or similar or even lower than fertility during heat outside this period. There are still some controversies on this matter, so we hypothesized that non-lactating mares have a greater potential for pregnancy rate than that lactating mares in foal heat, because the uterine involution is already finalized and ovarian activity is basically normalized in non-lactating mares, opening greater possibilities to became pregnant. In Southern Brazil, the Criollo breed (an original cross between Andalusian and Berber breeds) is a well stablished and widely used breed of horse, as it adapted well to the features of the Brazilian climate under field conditions, and developed two useful abilities (too much for work, and sports). To improve the reproduction mainly in Criollo horses, this study aimed to compare the pregnancy rate (PR) and other reproductive parameters (parturition-artificial insemination interval; parturition-ovulation interval; diameter of preovulatory follicle; artificial insemination-ovulation interval) between lactating mares in early puerperium and non-lactating mares artificially inseminated (AI) ≥ 180 days postpartum. This study took place at a stud farm in Southern Brazil between September and December (breeding season). Thirty-one mares Criollo breed, multiparous with mean age 9.6 years (range 6 -14) and body condition score (BCS) 3.1 (1= thin, 5 = obese) were divided into two groups: lactating mares (LMs) subjected to AI <10 days postpartum (n =16) and non-lactating mares (NLMs) AI ≥180 days postpartum (n =15). The mares were selected based on clinical criteria (absence of clinical endometritis or abnormal uterine discharge and by parturition date). Both groups had the same environment and nutritional conditions. The ovaries were examined by ultrasonography every other day; after detection of a follicle ≥ 30 mm in diameter (= the sum of the larger and smaller diameter divided by 2), the examinations with ultrasonography were conducted daily. After the detection of follicles ≥ 35 mm, uterine edema, and open cervix, ...