Parathyroid hormone (PTH) receptor binding property in the membrane fraction of the endometrium of the shell gland (uterus) of the guineafowl was analyzed by the use of [125I]PTH-related peptide (PTHrP) binding assays. Specificity, reversibility, and saturation of binding were demonstrated. Scatchard plots revealed a single class of binding sites. The equilibrium dissociation constant (Kd) was 0.50 to 1.15 nM in laying birds and 1.07 to 1.16 nM in nonlaying birds. The maximum binding capacity (Bmax) per milligram of membrane protein was 65.2 to 110.9 femtomoles (fmol) in laying birds and 105.8 to 120.6 fmol in nonlaying birds. Both Kd and Bmax values changed within the above range during an oviposition cycle in laying birds, showing a decrease during the period of eggshell formation. No change was found in nonlaying birds. The results suggest that PTH/PTHrP receptors are present in the shell gland of the guineafowl, and their binding may be related to eggshell formation.