The plasma half-life (T½) and target organ uptake of the subunits of hCG (hCG-α and hCG-β) were investigated. For these studies, tritiated (3H) hCG, hCG-α, and hCG-β were injected iv into intact immature female rats. At timed intervals plasma concentration was determined by specific homologous radioimmunoassays and liquid scintillation counting for 3H. T½ was also determined by radioimmunoassay for unlabeled hCG, hCG-α, hCG-β and recombined subunits, hCG-αβ. Tissue distribution (renal, hepatic, splenic, ovarian) versus time was determined by 3H counting. All substances had exponential disappearance curves with fast and slow components. The T½ (rapid) was 141, 6.2 and 11.1 min for hCG, hCG-α and hCG-β, respectively (p < 0.01) and T½ (slow) of 725, 58 and 81 min, respectively. Slopes of the tritiated and homologous unlabeled compounds were indistinguishable as were those of the native hCG and recombined subunits. 3H-hCG, 3H-hCG-α and 3H-hCG-β were rapidly concentrated in the kidney with lesser concentration in the liver and spleen. 3H-hCG was concentrated in the ovaries, whereas 3H-hCG-α and 3H-hCG-β could not be detected in the ovaries, correlating with their lack of intrinsic biologic activity. This lack of biologic activity and ovarian concentration is most likely due to the structural alterations of the hCG molecule which results from dissociation into its subunits. (Endocrinology91: 1030, 1972)