The goal of this study is to perform a comparative genetic investigation of testicle development during the postpubescence period (from days 70 to 90 of life) in the inbred mice lines PT and CBA/Lac. Interlinear differences in the body and testicular weight, serum testosterone concentration, number of epididymal spermatozoa, area of testicular epithelium, semeniferous tubule lumen, and insulae of Leydig cells were analyzed. It was found that the morphological and histomorphometric parameters of testicles in males from the PT line compared to the males of the CBA/Lac line did not reach a definitive stage with the end of the postpubescence period and kept on developing until day 90 of life. Therefore, genetic differences remain in the postpubertal testicular development of laboratory mice.