In order to determine the age at which Swiss-Webster mice first became capable of learning and exhibiting 24-h retention of a directional, one-way active avoidance response, six separate groups of 20 mice received 25 active avoidance training trials at selected ages between 11 and 21 days of age. Equal numbers of yoked-shock control mice received identical amounts of shock and handling as their trained littermates on each trial. Following a 24-h retention interval, all groups received 25 active avoidance training trials. The acquisition results indicated that, while all age groups were capable of emitting some successful avoidances, within-session improvement was not apparent until 17 days of age. Comparisons of retest scores by trained mice with their formerly yoked-shocked littermates revealed that reliably more avoidances were made by mice with prior training beginning at 17 days of age. In general, the results suggest that the ability to acquire and remember for 24 h an active avoidance response emerges around 15-17 days of age for Swiss mice on this directional task.