Random mated Swiss mice swam for 3 min at 20 degrees C. The animals with less than or equal to 10 s and greater than or equal to 50 s postswim latencies on the hot plate (56 degrees C) were selected as progenitors of low (LA) and high (HA) analgesia lines, respectively. Gradual divergence of latency distributions and postswim hind paw flick latencies between the two lines was observed in successive generations. As shown by the tail flick test applied to the sixth offspring generation, postswim analgesia in the HA line was not only more pronounced but also lasted longer compared to the LA line and the unselected controls. Possible differentiation of the endorphin system activity in the course of selection is discussed.