Six experiments traced the development of long-term memory of an escape response in the rat. The first showed 9 days to be the age at which rats first show an intersession improvement when sessions occur at 24-hr, intervals. The second showed the improvement to be a residual effect of training, and the third, that rats younger than 9 days display a shorter term retention. Experiments 4-6 provided a basis for referring to the 24-hr, retention as long-term memory. Collectively, they indicated that hypothermia selectively impairs 24-hr, retention, and that the degree of impairment is related to time between training and hypothermal treatment, findings which in research on adults are attributed to disruption of longterm memory processes.