SUMMARY Adrenal weight and body weight were measured for 100 mice of each sex of the strains: A/Cam, CBA/FaCam and SF/Cam. This was also done for reciprocal F1 hybrids between A and CBA, for reciprocal backcrosses, and for F2 hybrids. Using regression analysis, adrenal weights were corrected for differences in body weights. Sexes and strains were found to be significantly different with respect to both absolute and corrected adrenal weight. The males of the F1 cross between the A and CBA strains showed partial dominance of the A genotype. Reciprocal F1 females were found to differ significantly. This may be explained in terms of parent-genotype: offspring-genotype interactions. The variances of the heterogeneous stocks: F2, backcross of F1 to A and backcross of F1 to CBA, were found to be significantly higher than those of the homogeneous stocks: A, CBA and F1's. This indicated that at least some of the variation in relative adrenal weight between the strains was genetically determined. The frequency distributions of the ratio of adrenal weight to body weight in F2 and reciprocal backcrosses conformed to the classical pattern of a metrical character.