C-cell tumors (medullary thyroid carcinoma) occur in humans and several other mammalian species. This tumor develops spontaneously with a high incidence (50%) in old Wag/Rij (Wistar-derived strain) rats. We have recently shown that calcitonin binding sites, which are present in the Wistar rats, are lost from renal medulla of the Wag/Rij rats before they reach the age of 1 month. In the present work, we investigated the distribution of calcitonin binding sites in the kidneys of first and second generation hybrids of Wistar x Wag/Rij rats. The absence of calcitonin binding sites from the renal medullas of 25% of F2 hybrids indicates that the deficiency is inherited in a Mendelian fashion and opens the way to establishing inbred strains lacking renal medullary calcitonin binding sites.