Spontaneous alternation was examined in young rats following microinjections of anticholinergic agents into 4 hippocampal sites: anterodorsal or posteroventral dentate gyrus, hippocampus or entorhinal complex. The rate of alternation remained around 40% at 5, 10, and 15 days, increased suddenly to 80% on day 17, did not vary until day 20, regressed partly and temporarily until day 30, and returned to a near-adult level on day 40. Concomitantly with the transient regression of alternation between days 20 and 40, fear responses to environment were seen to emerge (boluses and squeaks), to reach a maximum on day 30, and to return to a low level by day 40. Injections of atropine (4, 8 micrograms) or scopolamine (4, 10 micrograms) into any of the 4 sites significantly reduced the rate of alternation from day 17 on. Only the highest doses were active at 10 and 15 days. These results demonstrate that spontaneous alternation and hippocampal muscarinic cholinergic mechanisms develop simultaneously and progress suddenly on postnatal day 17. Atropine and scopolamine also affected fear responses, abolishing or potentiating them according to the site of injection, showing that hippocampal cholinergic mechanisms exert complex influences on fear-induced emotional reactions.