SUMMARY The volumes of the adrenal cortex and medulla were measured for young adult male mice from the A/Cam and CBA/FaCam strains and from five hybrid generations produced by crossing these inbred strains. The hybrid stocks were the reciprocal F1's, the F2 and the backcrosses to both parental strains. Two independent sets of measurements were made. The second set confirmed and extended the results of the first. Both the adrenal cortex and the adrenal medulla were significantly larger in the CBA mice than they were in the A mice. It seems likely that the observed difference between the strains in cortical volume was determined by genetic variation either at a single locus or, at most, at two loci. The difference in medullary volume was determined by variation at at least two gene loci. In all cases the alleles found in the CBA mice showed dominance. The problems of deciding whether four separate loci were involved or whether all the differences were due to pleiotropic effects of fewer genes is discussed. In the first experiment measurements of the volume and number of cells in the zona fasciculata were made in addition to those of medullary and cortical volume. These measurements confirmed the existence of genetic variation in these characters which had been suggested by earlier findings of significant strain differences in them. The results of the second experiment also showed that the significant strain difference between A and CBA mice in testis weight was brought about by genes other than those causing the adrenal differences. The relation between the differences in adrenal structure described in this paper and those known to exist between these two strains in other aspects of the adrenal glands and in their target organs are discussed, as is the relation between findings on inbred strains and those to be expected in natural populations.