Mice treated with nicotine sulfate showed, as compared with saline-treated controls, a decreased incidence of active avoidance conditioning without effects upon either passive avoidance acquisition or escape behavior. The effect of nicotine was to reduce significantly the ratio of bound: free acetylcholine in the cerebral cortex. This change was accounted for by a decreased content of stored amine, particularly in the synaptic vesicles, without any change in the turnover of the “free” storage pool. A difference in the ratio of brain acetylcholine storage pools accounting for different modes of avoidance is suggested.