Summary. In cyclic rats, the highest ovarian specific binding for LH was 6·0±2·2% in pro-oestrus. During pregnancy, the specific binding of125I-labelled 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 was also highest in pro-oestrus (8·9±2·1%), decreasing to about 50% in oestrus and metoestrus, but staying relatively constant during pregnancy. For prolactin, the specific binding in rat ovaries was highest (7·1±1·6%) in pro-oestrus, quite high in metoestrus and dioestrus and low in oestrus. Specific binding increased gradually only after Day 14 of pregnancy. Serum concentrations of rat LH, FSH and prolactin at different stages of the oestrous 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 constants (Ka) for the hormones were very similar between ovaries from pro-oestrous rats and late-pregnant rats, being 0·31 × 109 M–1 for LH, 0·65 × 1010M–1 for FSH,and 1·14 × 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.