For evaluation of the effects of orchiopexy and orchiectomy on the contralateral descended testis, unilateral cryptorchidism was produced in 60 21-day-old mice by suturing of the left testis to the inner abdominal wall. A sham operation was performed on the left testis of 20 control mice (S). After creation of unilateral cryptorchidism, orchiopexy (P) or orchiectomy (O) was performed on the left testis at 2 (group I) and 10 (group II) weeks (ten mice each). All testes were removed for examination 2 weeks after orchiopexy or orchiectomy. In both groups the mean values of testicular weight, seminiferous tubular diameter, and tubular biopsy score of contralateral O testes were always statistically similar to the values of contralateral S testes (p greater than 0.01). The difference between TBS values of contralateral P and S testes was not significant in group I (p greater than 0.01), but it was significant in group II (p less than 0.01). The data from this murine experiment suggest that orchiectomy for unilateral cryptorchid testis is almost always helpful in improving contralateral morphology; however, orchiopexy can be helpful if it is not performed late in the period of maturation.