The recombinant inbred (RI) mouse group is useful for genetic analysis of a phenotype which is regulated by multiple loci. The female adrenal cortex was compared histologically to detect the strain differences between A/J and SM/J for the purpose of genetic analysis of adrenocortical morphogenesis using the SMXA RI group constructed between A/J and SM/J. Clear morphological differences were found in the zona reticularis and X zone in addition to the significantly small body and adrenal weights in SM/J. The z. reticularis was markedly thicker in SM/J than in A/J and the X zone was constituted of vacuolated cells in A/J and non-vacuolated cells in SM/J. The genetic control of these morphological traits was analyzed using SMXA RI strains. The results suggest that the thickness of the z. reticularis may be controlled by single locus and that vacuolation of the X zone by 3 different loci between A/J and SM/J.