The poor fertility after orchiopexy can be due either to the previously prolonged high abdominal temperature, to surgical trauma or to congenitally malformed or dissociated testis and epididymis. In order to test whether manipulation of the testis and the spermatic cord in juvenile dogs causes damage to the seminiferous epithelium after puberty, orchiopexy was performed stepwise in 31 normal young dogs. In four dogs, in which traction to the proper testis ligament or biopsy was carried out, a significantly lower relative testis weight was found, together with impaired differentiation of the seminiferous epithelium. From these experiments it was clear that surgical procedures in the inguinal and scrotal region do not necessarily affect the development of the seminiferous epithelium in the dog. However, trauma to the vaginal tunic seemed to be crucial, causing damage to the differentiation of the seminiferous epithelium.