Abstract. The granulosa cells from immature rats were found to possess binding components for progestins. The physicochemical properties were determined. By sucrose gradient centrifugation, binding of [3H]R-5020 was observed in the 7S and 4S region of the gradient. Isoelectric focusing, however, revealed only one binding component (pH 5.9). The component was of protein nature and was destroyed by heating the cytosol fraction to 37°C for 30 min. Scatchard analysis of the data showed one single binding component with high affinity (KD ∼ 1.5 × 10−9 mol/l). The binding capacity was 50 fmol/mg protein in unprimed animals and 193 fmol/mg protein in animals treated with oestrogen. Studies on the steroid specificity revealed that R-5020 had the highest affinity for the receptors, followed by progesterone and the synthetic progestin, cyproterone acetate. Corticosterone, oestradiol-17β and testosterone had low affinity, whereas cortisol had no significant affinity for the receptor. Using exchange assay, it was demonstrated that the cytoplasmic receptors translocated to the nucleus after administration of unlabelled progesterone to the animals. The presence of cytoplasmic and nuclear progestin receptors, strongly indicates that progesterone may have a direct effect on granulosa cell function.