Although surgical induction of cryptorchidism in the rat is known to cause infertility due to disruption of spermatogenesis, the exact cellular mechanism responsible for the degenerative changes in cryptorchid testes is unclear. Using a sensitive autoradiographic method for the detection of apoptotic DNA fragmentation, we have investigated the effect of experimentally induced cryptorchidism on apoptotic cell death in testes of immature rats. Bilateral or unilateral cryptorchidism decreased the weight of affected testes within 4 days; these decreases (24–27%) became significant (p < 0.05) at 7 days after the operation. Testes of sham-operated animals contained predominantly high molecular weight DNA (> 15 kb), whereas DNA cleavage into low molecular weight ladders characteristic of apoptosis was increased by induction of bilateral cryptorchidism in a time-dependent manner, i.e., 2.0-, 2.8-, and 4.2-fold (p < 0.05) at 2, 4, and 7 days after operation, respectively. In unilaterally cryptorchid animals, sham-operated testes also contained predominantly high molecular weight DNA, whereas induction of cryptorchidism of the contralateral testes increased DNA cleavage into low molecular weight fragments 3.0-, 2.8-, and 3.9-fold (p < 0.05) at 2, 4, and 7 days after the operation, respectively. In situ analysis of DNA fragmentation in testes of unilaterally cryptorchid rats at 7 days after the operation indicated that germ cells, mainly primary spermatocytes, were affected and that the percentage of seminiferous tubules containing labeled cells increased in the operated testis as compared to the contralateral control in the same animal. Furthermore, according to radioligand receptor analysis, specific [¹²⁵I]-hCG binding to cryptorchid testes was not affected at 2 and 4 days after the operation but was significantly (p < 0.05) decreased (32–35%), in relation to values in sham-operated testes in both bilaterally and unilaterally cryptorchid animals, at 7 days after surgery. In addition, testis [¹²⁵I]-hCG binding per milligram protein was not affected by the induction of bilateral or unilateral cryptorchidism throughout the 7 days of the experiment. Our data indicate that cryptorchidism-induced testis cell degeneration is mediated by apoptosis probably as the result of increases in testis temperature, leading to delayed decreases in LH receptors of Leydig cells. Although apoptotic cell death induced by bilateral cryptorchidism might be affected by changes in systemic factors, the increase of apoptosis in male germ cells after unilateral cryptorchidism is presumably regulated by local testicular factors. Experimentally induced cryptorchidism provides a useful model for study of the role of elevated temperature on testis cell apoptosis.