Hibernating European hamsters reacted differently to sc injections of N-nitrosodiethylamine (DEN) than did European hamsters that were not hibernating. Hibernating animals tolerated higher dose levels but developed fewer neoplasms. In contrast, hibernating males had more pulmonary tumors than did the respective nonhibernators. However, the hibernating females of the low dosage group developed fewer lung tumors. The survival times were longer for the male hibernators than for the male nonhibernators. The organ specificity of DEN, as well as the morphology and histogenesis of the neoplasms, showed no differences between the hibernating and nonhibernating groups.