Alpha and LH beta subunit mRNAs were measured in pituitaries of 4-day cycling rats during the estrous cycle. A two-fold increase in alpha mRNA occurred between 0800-2000 h on diestrus, but alpha mRNA concentrations were stable during other days of the cycle. LH beta mRNA concentrations were low during estrus and metestrus (11-16 pg cDNA bound/100 micrograms pituitary DNA), but were elevated (27-30 pg) between 0800-2000 h on diestrus. A second increase in LH beta mRNA was observed on the afternoon of proestrus, prior to the onset of the LH surge with maximum values (45 pg) coincident with peak LH secretion. LH beta mRNA concentrations declined rapidly and had fallen to basal values by midnight on proestrus. These data show that alpha and LH beta mRNAs change in a similar manner during metestrus, diestrus and estrus, suggesting coordinate regulation of alpha and LH beta gene expression at these times. During the LH surge, however, LH beta mRNA alone is increased, suggesting that the LH beta gene can be differentially expressed at times when maximum LH secretion is occurring.