In the reproduction of animals and humans, insulin-like factors (IGF-I , IGF-II) play a significant role. Among others, they stimulate growth of the ovarian follicle, embryo development, egg implantation and they inhibit cell apoptosis. In horses the implantation process and placenta development is partly regulated by IGF-I and IGF-II. Research was conducted on 18 mares during early pregnancy. The study has revealed the existence of some differences in the level of IGF-I and IGF-II. A significant increase of IGF-I in comparison to the preovulation period (311 ng/ml) was noted 12 h after ovulation (356 ng/ml), 72 h (328 ng/ml), 7 days (340 ng/ml) and at 35 (344 ng/ml) and at 55 (360 ng/ml) days of gestation. The concentration of IGF-II also increased but only at the 6th day after ovulation. The concentration of IGF-II before ovulation was 4.8 ng/ml and up until the 6 days after ovulation it ranged from 8.2 to 9.6 ng/ml. The differences in the levels of IGF-I and IGF-II before and after ovulation and during the pregnancy could result from the activation of an embryo genome and from the preparation of endometrium for implantation.