Kidney androgen-regulated protein (KAP) is a unique protein of unknown function that is transcriptionally induced by sex steroids. KAP is thought to be predominantly a kidney-specific gene. After conducting a differential screen of a mouse uterus cDNA library, a clone was identified that is identical to KAP. Using this cDNA to generate radiolabeled cRNA probes, Northern blots were conducted against the following tissues collected sequentially during the latter third of pregnancy: kidney, uterus and placenta. Abundant message was present in all samples of the kidney tested and there was a slight, but apparent, increase (1.5-fold) in expression during the period surrounding birth. Message is also present in the uterus, at levels comparable to the kidney, but expression occurs only during the period surrounding birth. Message is not present in the uterus at any other time. Message is also not detected in the placenta or in several other tissues tested. In addition to the kidney, KAP gene is also transcribed at equivalent levels in the uterus. Unlike the kidney, expression in the uterus is limited to the perinatal period.