The density of the spinal cord thyrotropin-releasing hormone (TRH) receptor in male and female rats was measured during postnatal development. The younger rats of both sexes (11-16 days old) exhibited higher levels of TRH-binding than did adults. A comparison of the properties of receptor binding for young and adult spinal cords showed that the increased binding resulted solely from an increase in maximum binding capacity rather than a change in receptor affinity. Furthermore, no sex-dependent difference in the receptor property was found at any stage of development. In conclusion, these findings demonstrate that the differences in the maturation of TRH receptor itself may not be the cause of known gender-specific differences in the physiologic actions of TRH in the spinal cord of young rats.