In addition to the known four alternative first exons E11, E12, E13 and E14 of the rat prolactin receptor (PRL-R) gene, a novel first exon, E15, was identified by cDNA cloning of the 5′-end region of PRL-R mRNA in the rat liver. Genomic fragments containing E15 and its 5′- or 3′-flanking regions were also cloned from rat kidney genomic DNA. A sequence search for E15 revealed that E15 is located 49 kb upstream of exon 2 of the PRL-R gene in rat chromosome 2q16. RT-PCR analysis revealed that E15 was preferentially expressed in the liver, brain and kidney. Expression profiles of E12-, E13- and E15-PRL-R mRNAs in the liver of male and female rats at 5 days of age and those at 8 weeks of age were examined by RT-PCR. The levels of E12-PRL-R mRNA in the female rat increased remarkably in rats at 8 weeks of age compared with those at 5 days of age, and the levels of E15-PRL-R mRNA in the male rat decreased markedly at 8 weeks of age compared with those at 5 days of age. In the female rat, the levels of E12-PRL-R mRNA at 8 weeks of age decreased with ovariectomy performed at 4 weeks of age and recovered with the administration of β-oestradiol. On the contrary, the levels of E15-PRL-R mRNA increased with ovariectomy and decreased with the oestrogen treatment. In the male rat liver, the levels of E12-PRL-R mRNA at 8 weeks of age increased strikingly with castration performed at 4 weeks of age and became undetectable with the administration of testosterone. The levels of E15-PRL-R mRNA increased slightly with castration and were restored by testosterone treatment. Removal of gonadal tissues and sex steroid hormone treatment had no effect on the expression levels of E13-PRL-R mRNA in both female and male rat livers. These results indicated that the expression of the PRL-R gene in the liver is regulated by the differential effects of sex steroid hormones on the transcription of the multiple first exons including the novel one.