The objective of this study was to determine the biochemical characteristics, subtypes, and localization of melatonin receptors in the rat uterus in estrous stage. Autoradiography with the melatonin ligand, 2-[125I]iodomelatonin, showed that melatonin receptors were localized in the rat uterine endometrium. Binding of 2-[125I]iodomelatonin in crude membrane preparations of rat uterine endometrium in estrous stage was stable, saturable, reversible and of high affinity. Rosenthal analysis yielded an equilibrium dissociation constant (Kd) of 28.9 +/- 3.59 pmol/l (n = 8) and a maximum number of binding sites (Bmax) of 1.6 +/- 0.15 fmol/mg protein (n = 8). The Kd value determined from kinetic analysis was 16.5 +/- 3.02 pmol/l (n = 3). Competition studies using various indoles and neurotransmitters demonstrated that 2-iodomelatonin, melatonin, 6-chloromelatonin, 6-hydroxymelatonin and N-acetylserotonin showed significant inhibition of the 2-[125I]iodomelatonin binding, while the other indole compounds tested had no significant inhibition. The expression of rat uterine endometrial melatonin receptor subtypes was studied by reverse transcription-polymerase chain reaction (RT-PCR) using mt1 and MT2 receptor gene-specific primers. mt1 receptor cDNA was amplified and confirmed by nucleotide sequencing. These findings indicate that mt1 receptors were present in the rat uterine endometrium, and suggest that melatonin plays an integral part in uterine physiology.