Five experiments with wild and domestic rats (Rattus norvegicus) were designed to assess behavioral effects of domestication. The results, which must be considered preliminary, are supportive of the notion that domestication in the Norway rat has induced an adaptive fitness rather than degeneration: 1. Scar-marked captives showed the poor reproductive success typical of traditional attempts at initial laboratorization. In 10 such pairs, three of the females killed prospective mates and five of the six who produced litters cannibalized all or some of the offspring. Unscarred captives, assumed to represent higher social status in the wild population, produced litters in eight of ten pairings with no abnormal breeding or maternal behaviors. 2. Scar-marked captives were least dominant in water competition and were most likely to show abnormal behaviors, new wounds, and mortalities. Similar competitive groupings of domestic rats showed the expected difference in social behavior; fighting was less intense, social distance was less, and threat signals were rare compared to wild rats. Where wild and domestic rats were housed together, however, the intensity of social behaviors in the latter increased, even leading to deaths, although interaction was almost segregated between the two kinds of rats. 3. In an avoidance learning situation, wild-caught and wild F1 (laboratory raised) rats were markedly inferior in performance compared to domestic rats in paradigms with internally or externally cued warnings. 4. In another avoidance learning context, wild rats showed limited evidence of a passive style of learning while domestic rats showed clear evidence of active avoidance learning. These data suggest that learning styles may differ in the two kinds of rats and that definitive evidence of a learning difference will be difficult to obtain. 5. Daily patterns of responding for drinking water were more resistant to change in the wild rats, even at a substantial cost in water obtained, suggesting that domestication may have induced greater behavioral flexibility.