The normal sibling rats of the infertile male were selected for brother-sister breeding.The stocks of every filial generation whose male siblings were infertile were inbred with their brothers or sisters by optimum seeking method.After 20 generations,a rat inbred strain of spontaneous male infertility was established,and was named MIJ.The genetic homozygosity was analyzed by skin grafting.The phenotypes of MIJ were observed,and the mode of infertility inheritance was studied by test cross experiments.The results demonstrated that MIJ rats had reached the norm of inbred strain,and the infertile male rats showed the phenotypes of delayed testicular descent,testicular and epididymis hypoevolutism,oligozoospermia and hypospermatogenesis.The phenotypes of the other rats in the inbred strain were normal.The infertility mutation was inherited in a recessive mode,and was controlled by a monogenic recessive gene in euchromosome.The infertile male rats could serve as a useful model for the study of human infertility.