The purpose of this research was to examine the hormonal regulation of the PRL receptor (PRL-R) gene expression in the mammary gland of early pregnant mouse. Following reversetranscription, the quantity of PRL-R mRNA was determined by the competitive polymerase chain reaction. The level of long form PRL-R (PRL-R_L) mRNA changed cyclically with the highest at estrus and the lowest at diestrus II. PRL-R_L mRNA was maintained at high levels for the first 3 days of pregnancy but declined to lower levels on day 5. Mice ovariectomized on day 2 of pregnancy maintained the same level of PRL-R_L mRNA during the 24h-period. The level of PRL-R mRNA increased more than 2- and 2.7-fold with 17β-estradiol and PRL, respectively, and the progesterone concentration decreased its levels to 71% of the vehicle-injected control. The increasing action with 17β- estradiol and PRL was suppressed by administration of progesterone. Mice ovariectomized on day 3 had a 1.8-fold higher level than that of the sham-operated control. The short form of PRL-R remained at low levels throughout the experiments. The results suggested that the expression of the PRL-R gene was suppressed when serum progesterone increased during early pregnancy.