The endocrine regulation of parturition exhibits several interspecies differences. Among the endocrine parameters considered to be the most important ones from the point of view of the regulation of parturition the concentration of oestrogens (oestradiol, oestriol and oestrone), progesterone, PGF2 alfa and its degradation product PGFM, was determined in both peripheral and uterine venous blood by RIA method in rats starting from the 15th day of pregnancy to the 3rd or 4th day following parturition. In the pregnant rat oestriol could only be detected on the day of parturition (21th day). Concentrations of oestriol and oestradiol were the highest on the day of parturition. An opposite tendency could be observed as far as progesterone concentration was concerned, i.e., the concentration decreased gradually from the 15th day of pregnancy onwards and the lowest value was reached on the day of parturition both in peripheral and uterine venous blood. PGF2 alfa and PGFM concentrations in the uterine vein increased gradually from the 15th day of pregnancy and the highest value could be detected peri partum. Our data, in accordance with those of others support the idea that the hormones investigated are involved in the process of parturition, i.e., characteristic changes of oestrogens, progesterone and prostaglandin levels ensue in the rat prior to parturition.