The binding of 125I-labelled human chorionic gonadotropin (HCG) was studied using thick slices (300 μm) of rabbit ovarian tissue. Binding was saturable, reversible, stereospecific, and of high affinity. The amount of binding was proportional to the number of slices used and could be destroyed by boiling. Ovarian slices from eight individual rabbits were found to have two binding sites for 125I-labelled HCG with KD values of 272 ± 64 and 1263 ± 274 pM and Bmax values of 25.7 ± 5.3 and 94.1 ± 18.8 fmol/mg protein, respectively. In a comparative study the KD and Bmax values were 351 ± 151 pM and 25.3 ± 11.1 fmol/mg protein with slices from one ovary and 134 ± 24 pM and 109 ± 32 fmol/mg protein with membranes from the contralateral ovary. These data suggest that the binding of HCG can be determined in live tissue.