One-hundred-twenty-nine cases of adrenal hyperplasia in the rabbit were shown to be associated with a fully penetrant autosomal recessive gene, symbolized ah. The mutant is viable prenatally, may be recognized by gross examination of the adrenal at least as early as 19 days of gestation, but dies soon after birth. There is an external feminizing effect of the gene and the model is similar to the feminizing congenital lipoid adrenal hyperplasia reported in man. This mutant is being maintained in strain IIIVO/ahJ.