In order to examine the time-dependent expression of the PRL receptor (PRL-R) gene at lactogenesis, the level of PRL-R mRNA was determined following ovariectomy in pregnant mouse mammary gland. Following reverse transcription, the quantity of mRNA was measured by the competitive polymerase chain reaction. The casein (a 22000 molecular weight component) mRNA level was measured as a marker for milk synthesis. Following ovariectomy, the onset of abortion occurred mostly at 22-23h and the level of casein mRNA began to increase at 12h. The long and short forms of PRL-R mRNAs were detected in a molar ratio of 1:0.2 on day 12 of pregnancy. Eight h after ovariectomy, the long form of PRL-R mRNA began to increase, showing a bell-shaped profile with the highest peak at 16h. The short form of PRL-R mRNA was at low levels and remained constant. The levels of the long form of PRL-R mRNA decreased similarly in the presence and absence of foster pups from 24 to 48h. Conversely, casein mRNA were maintained at high levels by supplying foster pups. The level of the long form of PRL-R mRNA reached a maximum prior to abortion. The present experiments demonstrated that the acute expression of the PRL-R gene occurred in the mammary gland at lactogenesis.