125I-labeled hCG (human [125I]iodoCG) was injected iv into male and female rats, and the disappearance of the hormone from the circulation was studied. Also, the uptake of the hormone by different tissues, as well as its catabolic fate after the initial uptake were analyzed. The disappearance curve of human [125I]iodoCG consisted of two exponential components with apparent half-lives of 30 min and 4.8 h in both sexes. Two hours after the hormone injection, the uptake of radioactivity was 10.5% of the injected radioactivity in the kidneys, 2.2% in the liver, and 1.9% in the ovaries. A considerable amount of radioactivity, (1.5% of the total injected radioactivity) was also excreted in urine. Autoradiographs prepared from the kidney cortex and ovaries showed that renal uptake of the hormone took place mainly in the proximal tubule cells, and the ovarian uptake took place in the corpora lutea and theca cells. Gel filtration analyses of the radioactivity extracted with 0.5% Triton X-100 from the kidneys and liver at 30 and 120 min after hormone injection revealed three radioactive peaks with Kov values of 0.52, 0.71, and 1. The first compound was identified as labeled hCG and the second one as the a-subunit of the hormone. The low molecular weight compound was identified as [125I]- iodotyrosine using copper-Sephadex G-25 gel filtration. Three radioactive peaks with identical elution patterns were also obtained from urine 30 and 120 min after hormone injection. However, the low molecular weight compound was found to be free radioiodine. In plasma, most of the radioactivity appeared as intact labeled hCG at both times after hormone injection. These results suggest that the kidneys, liver and ovaries are mainly responsible for the removal of human [125I]iodoCG from the circulation in female rats. There is also some evidence that the hormone is split into subunits and hydrolyzed to free amino acids after its initial uptake.