Binding of 125I-labelled HCG to rat ovary varies during the oestrous cycle. The highest amount of binding was found in dioestrous-II rats (7.33 × 10−15 mol of HCG per mg wet weight), while the lowest binding was observed in oestrous rats (1.67 × 10−15 mol of HCG per mg wet weight). An inverse correlation exists between the binding capacity of the ovary and the concentration of circulating LH. It is concluded that the low binding capacity of the rat ovary in the oestrous stage is due to masking of the specific receptors by the excessively high LH hormone secretion in pro-oestrus.