The present investigation was undertaken to detect whether protein phosphorylation levels of hippocampal synaptosomes in mice trained on shuttle box avoidance task would show some changes. The synaptosomes were prepared at different times after the conclusion of training and labelled with gamma-32P-ATP for in vitro determination. The results are as follows: (1) No significant differences were found in the phosphorylation of synaptosomal proteins in different brain areas in normal mice before training. (2) Immediately after the conclusion of shuttle box avoidance task there appears a correlation between protein phosphorylation levels in hippocampal synaptosomes and the degree of success of training. (3) 24 h after the completion of training the above relationship is more significant, and protein phosphorylation levels of right hippocampus in Good Score Group is markedly higher than that of left one, but in the Inferior Score Group, the left-right difference is reverse. The formation of such asymmetry in protein phosphorylation in hippocampi is unknown. These data indicate that the synaptic protein phosphorylation in hippocampi might be a molecular mechanism of acquisition.