Epidermal growth factor is known to have mitogenic effects in the ovary. To determine whether this effect is receptor-mediated, and whether the receptor number changes during the estrus cycle, in this study we have evaluated the total and occupied EGF receptor (EGFr) concentrations and receptor binding characteristics in adult rat ovaries collected at diestrus and early estrus. These represent stages before and after the pituitary LH/FSH surges of the estrus cycle. The 100,000 g plasma membrane fractions were isolated and EGFr concentrations evaluated with and without 1 mM EDTA treatment at pH 4.5 in order to release receptor-bound ligand and therefore assess the unoccupied and total receptor concentrations. EGF receptor affinity and number were analyzed by Scatchard analysis. No statistically significant difference between unoccupied and total receptor concentration in the diestrus phase ovaries was found. The percentage of occupied receptors (total-unoccupied) in diestrus membranes was 7.1%. In early estrus ovaries, a significant increase (P < 0.05) was observed in total receptor concentration when compared with the number of unoccupied receptors. The percentage of occupied receptors in early estrus membranes was 35.7%. Comparison between the ovaries from the two phases indicated an almost twofold increase in total EGF receptor number in early estrus membranes when compared with diestrus membranes. Furthermore, the percentage of occupancy between the two studied groups indicate a significant increase (P < 0.05) in the number of occupied receptors in early estrus when compared to diestrus. Thus, we have demonstrated that not only EGF receptor concentrations are modulated through the rat estrus cycle, but also the concentration of EGF-like substances bound to such receptors.