[The role of progesterone and its receptors in the development of the uterotropic reaction in ovariectomized rats].

In intact rats, the progesterone level in cytosolic fractions of various organs did not correspond to the level of the progesterone receptors. In ovariectomised animals receiving progesterone, the hormone accumulation in the uterus was fulfilled without participation of progesterone receptors. Over half of the progesterone in the rat uterine tissues exists in receptor-unbound "free" form or, possibly, in the form of complexes with small molecular weight. The uterotropic effect of progesterone in ovariectomised animals seems to be determined by the presence of the hormone in the "free" form in the uterus cells or, possibly, in the form of low-molecular complexes.