The effects of inhibitors of RNA or protein synthesis(cycloheximide,puromycin and actinomycin D)on acquisition of learning in mice were investigated using various types of shock avoidance tasks. Mice having uniform and superior learning ability were used as subjects. Drugs were injected into the lateral ventricle 1hr before training for avoidance tasks in three different shapes of maze and in a one-way shuttle box and for those based on light-dark discrimination.In all cases of training,the drugs were found to impair the acquisition of learning.The impairment was pronounced in a relatively complicated task such as H-maze learning as compared to a simple task such as T-maze learning.The drugs never affected the performance of pretrained animals.During the discussion,the acquisition of learning would be related to some specific process of RNA and protein synthesis in the brain.Up to now much effort has been made to discover the neurobiological processes that underlie learning and memory.Previous studies have indicated that cycloheximide,puromycin and actinomycin D impair the formation of long-term memory,but they do not affect the acquisition of learning or the formation of short-term memory (AGRANOFF et al.,1965;BARONDES and COHEN,1967;FLEXNER et al.,1967;AGRANOFF,1968;COHEN and BARONDES,1968).These results suggest that the processes of synthesis of macromolecular substances such as protein and/or RNA in the brain do not mediate learning at least in its acquisition stage. However, SQUIRE et al.(1973)reported that a slight divergence of learning curves occurred between control mice and cycloheximide-injected ones at an advanced stage of training in a discrimination task,suggesting that the drug may affect the acquisition of learning.Another approach to clarify the neurochemical mechanisms involved in learning and memory was to demonstrate changes in the level of macromolecular substances (HYDEN and EGYHAZI,1962;HYDEN and LANGE,1965;ADAIR et al.,

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