Our previous reports have revealed the postnatal developmental pattern of progesterone receptor (PR) in the rat cerebral cortex, but little is known about PRmRNA changes in the tissue so far. In the present study, we have attempted to detect and quantify PRmRNA in the tissue by the use of the reverse transcription-polymerase chain reaction (RT-PCR)-dot blot analysis. Cerebral cortical tissues were dissected from female Wistar rats at 2 and 8 days of age. Total RNA extracted from the tissues was reverse transcribed, followed by PCR. The PCR primer set, whose sequence was derived from the human clone, flanked the part (320 bp) of the human PRcDNA which corresponded to the progesterone binding domain. The 320 bp of the RT-PCR product was generated from rat uterine RNA. The amino acid sequence deduced from the nucleotide sequence of the product had a 95.5% identity with the corresponding region in human PR, confirming that the product had originated from the rat PRmRNA. Moreover, a highly sensitive and quantitative assay for rat PRmRNA had been developed by the use of RT-PCR-dot blotting. The PRmRNA was detectable in the cerebral cortex of both 2- and 8-day-old rats by the assay. Furthermore, the level of cortical PRmRNA of the 8-day-old rats were greater than the 2-day-old rats. These results indicate that the RT-PCR assay is useful for detection and quantification of PRmRNA in the neonatal rat cerebral cortex, and that increment of the cortical PRmRNA in the 8-day-old rat is associated with a drastic change of the level of the cortical PR around day 10.