The levels of mRNA for long and three short forms of prolactin receptor (PRLR) were examined in the livers of normal (db+/db-) and insulin-resistant diabetic (db+/db+) mice to assess the role of gonadal steroid hormones in the regulation of PRLR gene expression in diabetes mellitus. In females, plasma levels of testosterone in diabetic mice were higher, and those of 17beta-estradiol were lower when compared with levels in normal mice. By contrast, diabetic male mice had lower plasma levels of testosterone than normal males and showed no significant difference in the low circulating level of 17beta-estradiol compared with normal males. The short 3 form of PRLR (PRLR3) mRNA was the most abundant in the liver of both normal and diabetic mice. In addition, the level of PRLR3 mRNA in normal females was 8-fold higher than in normal males. The level of PRLR3 mRNA in diabetic females was approximately a quarter lower than in normal females, whereas the level of PRLR3 mRNA in diabetic males was approximately 2-fold higher than in normal males. During postnatal development, the level of PRLR3 mRNA increased during puberty in normal females, while the level in diabetic females decreased to a nadir at 7 weeks of age followed by a progressive rise. On the other hand, the levels of PRLR3 mRNA in both normal and diabetic males decreased gradually during 5 to 14 weeks of age. Testosterone treatment of diabetic males and females resulted in a 49.1 and 49.8% decrease of PRLR3 mRNA respectively. 17beta-Estradiol treatment slightly (18%) increased levels of PRLR3 mRNA in diabetic males. These results suggest that the hepatic level of PRLR mRNA is regulated by the inhibitory effect of testosterone and the stimulatory effect of estrogen in both normal and diabetic mice.