The binding of rANP(1-28) to receptors was studied on crude adrenal membranes from fetal rats between day 17 of gestation and term and also neonatal rats between weeks 1 and 4. The binding assays were carried out using 125I-rANP(1-28) as radioligand incubated with membrane preparations (2 mg/ml) for 90 min at 22 degrees C. The binding was specific, saturable and reversible. The Scatchard analysis of the binding data revealed a single class of binding sites of high affinity (kd approximately 10(-10) mol/l) which did not change significantly at all stages of development studied. The binding sites presented a higher affinity for ANP analogues which contained the C-terminal phenylalanine arginine residue. The number of ANP receptors expressed per adrenal increased regularly in fetal and neonatal rats and the perinatal evolution of these concentrations of ANP receptors was related to the increase in the size of the adrenals. When the concentrations of ANP receptors was expressed per microgram DNA, the concentrations of ANP receptors were higher in neonatal rats than in fetal rats and reflected the number of receptors per cell. These results suggest that these binding sites mediate the biological actions of ANF in the adrenal gland during the perinatal period.