In order to elucidate the mechanism of germ cell degeneration in experimental cryptorchidism, we examined the testes of adult mice from a morphological standpoint. Adult ICR mice were made cryptorchid either unilaterally or bilaterally. In some mice, testes were surgically replaced back into the scrotum at 2 months after induction of cryptorchidism to observe the regenerative process. Morphological changes of cryptorchid and replaced testes have been studied by light and electron microscopy. Testes were also examined by the TUNEL (TdT-mediated dUTP-biotin nick end labelling) method to evaluate whether the degenerative cells, spermatocytes and spermatids, were dying by apoptosis or by any other process(es). At 8 weeks after the induction of cryptorchidism, the seminiferous epithelium consisted only of Sertoli cells, spermatogonia, and some spermatocytes of early meiotic stages. Soon after the replacement of testes to the scrotum, most of the seminiferous tubules resumed spermatogenic processes. Many degenerating cells, especially the spermatocytes, showed condensation of the nucleus and cytoplasm in cryptorchid testes. Although the cytoplasm was markedly eosinophilic under a light microscope to imply condensation of the cytoplasm, the extent of the condensation was not as pronounced under an electron microscope as reported in previous publications. The cytoplasm showed no expansion. By the TUNEL method, many of the degenerating cells, mainly the spermatocytes, have been shown as undergoing apoptosis. These data provide evidence that at least some of the cells die by apoptosis, or by a process similar to apoptosis.