Laboratory rats of the Wistar strain were selectively bred for a high or a low level of short-term activity (+A, -A) and defecation (+D, -D) in a novel environment. In lactating females of groups +A-D, +A+D, -A+D and -A-D maternal behavior was tested in situations, differing in complexity and demands of the ability of the females to adapt themselves to a new environment. Most intergroup differences were dependent on the characteristics of their activity, while defecation manifested itself less markedly in maternal behavior. In all tests +A groups showed a higher number of active contacts with the young, and the duration of maternal activities was higher in them when compared with -A females. The results indicated difference in the structure of maternal behavior and in the strategy with which they coped with a new situation. Females -A arranged the scattered pups into a nest more rapidly but they manipulated less with them and retrieved them minimally. In -A more often than in the +A the cycle of maternal behavior was interrupted by the elements of other than maternal programs. On the other hand, females +A gave more attention to each pup, they carried it from place to place for a long time, they tended to rearrange the newly formed nest, maternal program being interrupted less often. The most marked difference appeared between groups +A-D and -A+D.