Changes in the distribution of the in vitro uptake of 125I-HCG by the ovaries of adult rats were examined histochemically throughout the estrous cycle. Only in follicles wider than 500 micrometer, occurring mainly at diestrus and proestrus, could granulosa cells bind the labelled hormone. The labelling increased with follicular size and decreased in intensity from the peripheral granulosa cells inwards. No uptake occurred in the oocytes, in the cells of the cumulus oophorus nor in the granulosa cells of the atretic follicles. The binding capacity of the newly-formed corpora lutea of estrus was less than that of preovulatory follicles. The uptake of 125I-HCG by corpora lutea during the first cycle reached its maximum at diestrus but fell sharply by proestrus. The uptake was patchy in the corpora lutea of the second cycle and not significant in the older ones. The uptake of 125I-HCG by thecae increased with follicular size and was greater in the thecae of atretic follicles than in the thecae of growing follicles of like size. There was a greater uptake in the last formed interstitial tissue than there was in older tissue. At proestrus, the uptake of 125I-HCG was unaffected by the LH surge at 18.00 h but had decreased slightly at 24.00 h. The implications of these data in relation to the regulation of receptor sites, is discussed.