The authors present developmental evidence that contextual fear conditioning is supported by a short-term memory system that supports conditioning immediately after a shock and by a long-term memory system that supports contextual conditioning 24 hr after training. This is based on the finding that after 1 conditioning trial, rats 18 to 32 days old show the same amount of conditioned freezing when tested immediately after conditioning but 18-day-old rats show much less conditioned freezing than the older rats when the retention interval is 24 hr. The data also suggest that the long-term memory representation of context that mediates conditioned fear is not available until several hours after the conditioning trial. Implications of these findings for memory consolidation processes, infantile amnesia, and hippocampal formation development are discussed.