In cyclic rats the highest ovarian specific binding for LH [luteinizing hormone] was 6.0 .+-. 2.2% in proestrus. During pregnancy the specific binding of 125I-labeled bovine LH by rat ovaries increased gradually and reached a maximum of 24.1 .+-. 4.9% between days 14 and 18 of gestation; a slight decrease in binding was observed at day 20 of pregnancy. Ovarian specific binding for FSH [follicle-stimulating hormone] was also highest in proestrus (8.9 .+-. 2.1%), decreasing to about 50% in estrus and metestrus, but staying relatively constant during pregnancy. For prolactin, the specific binding in rat ovaries was highest (7.1 .+-. 1.6%) in proestrus, quite high in metestrus and diestrus and low in estrus. Specific binding increased gradually only after day 14 of pregnancy. Serum concentrations of rat LH, FSH and prolactin at different stages of the estrus cycle and during pregnancy were determined by radioimmunoassays, and no obvious correlation was observed between levels of circulating hormones and the specific binding of these hormones in ovarian tissues. Affinity constant (Ka) for the hormones were very similar between ovaries from proestrus rats and late-pregnant rats, being 0.31 .times. 109 M-1 for LH, 0.65 .times. 1010 M-1 for FSH, and 1.14 .times. 1010 M-1 for prolactin. Increases in specific binding for different hormones were due to increases of total binding sites in the ovary under different physiological states.