Serum follicle-stimulating hormone (FSH), pituitary FSH content and FSH beta subunit mRNA concentrations were measured at 1 to 3h intervals throughout the 4 day estrous cycle in rats. Serum FSH was stable (range 200-320 ng/ml) apart from the biphasic proestrus surge (5 fold elevation) which was present from 1800h of proestrus through 0800 h on estrus. Basal FSH beta mRNA concentrations from late metestrus through the afternoon of proestrus were 0.10 +/- 0.04 f mol cDNA bound/100 micrograms pituitary DNA. The major increase in FSH beta mRNA began at 2000 h on proestrus, 2 h after the initial rise in serum FSH and peak mRNA concentrations (0.43 +/- 0.08 f mol cDNA bound) occurred at 0200 h on estrus. FSH beta subunit mRNA concentrations were again increased at 2300 h on estrus (peak 0.24 f mol cDNA bound) and remained elevated through 1700 h on metestrus. Pituitary FSH content was transiently increased during metestrus and diestrus, but was elevated at 1000 h through 1900 h on proestrus (peak 5-fold increase). FSH content fell rapidly at 2000h and remained low until 1400 h on estrus when values again rose. These data show that FSH beta mRNA is increased 4-5 fold during the proestrus FSH surge, and a smaller increase occurs on metestrus in the absence of elevated FSH secretion. The increased concentrations of FSH beta mRNA occurred at different times to the previously reported changes in alpha and LH beta mRNAs. Therefore, the data suggest that different mechanisms are involved in the regulation of LH and FSH beta subunit gene expression during the 4-day estrous cycle in rats.