When singly housed under laboratory conditions, male Syrian golden hamsters routinely attack novel conspecific intruders introduced into their home cages. As we report here, after being repeatedly defeated by a larger, more aggressive intruder, such normal territorial aggression on the part of the resident hamsters is replaced by defensive behavior and flight. We have found that such conditioned defeat (CD) can be reliably induced by a series of 5-min trials with an aggressive intruder whether these trials are spread over 4 days or are all given on the same day. A useful behavioral criterion for the appearance of CD during acquisition is the first occurrence of anticipatory flight (AF), i.e., the first time the resident flees from the next aggressive intruder before being attacked. CD shows generalization: Animals trained to the AF criterion (AF Group) subsequently show defensive behavior toward, and even flee from, intruders which show absolutely no sign of aggressiveness. Animals in the AF Group persisted in such defense behavior for two test sessions; animals given three additional defeat trials beyond the appearance of AF (AF + 3 Group) showed a greater magnitude and persistence of defense and flight. A comparison of CD-trained animals which met a nonaggressive intruder (NAI) every day for 5 days to similarly trained animals which met the intruder only on the fifth day after acquisition suggests that CD diminishes passively as a function of time and not as the consequence of repeated encounters with a nonaggressive stimulus animal. We also found that near ideal NAIs could be prepared by treating nonaggressive hamsters with high doses of diazepam: animals so treated locomote more or less continuously around the cage virtually ignoring the subject. An unexpected observation was that subjects in the AF Group tended to closely follow these diazepam-treated, rapidly locomoting NAIs around the cage. Following may be an example of the "risk assessment" activities directed toward a potential threat. The development of a rapid and reliable technique for inducing CD in hamsters sets the stage for further physiological and pharmacological work on this interesting phenomenon.