To determine the effect of suckling on the prolactin receptor (PRL-R) gene expression, we measured the quantity of PRL-R mRNA in the lactating mouse mammary gland. When the pups were separated from their mother on day 5 of lactation, the long form of PRL-R (PRL-R[L]) mRNA disappeared with a half-life of 12.5 h for the first 9 h and 3.0 h for the following 9-15 h. By supplying pups to mice which had been weaned for 24 h, PRL-R(L) mRNA increased 2.5-fold during the next 6 h-period. The increase in PRL-R(L) mRNA was found in the mammary glands from which the pups removed milk. The number of mammary PRL-R protein decreased or increased following weaning or following the removal of milk by suckling, respectively. From these observations. it was concluded that the removal of milk acutely increases the level of PRL-R(L) mRNA during lactation.