The effect of dietary condition on the acethylcholine and chotinesterase activity in the brain was observed. Rats weighting $120{sim}130;gm$ was fed on diets deprived of protein, carbohydrate and lipid, respectively. Experimental terms was devided into 3, 7 and 15 days. The results are summarized as follows: 1. The lipid free diet makes greater contribution to the gain of body weight than the carbohydrate free and protein free diet. 2. The acethylcholine in the protein free diet group is reduced after 7 days and that in the carbohydrate free diet, after 3 days. 3. The cholinesterase activity shows continuing decrease after 3 days by protein free diet, and the other diet groups appear temporal decrease at 7 days and increase, at 15 days. 4. The relation of acethylcholine, and cholinesterase is similar to each other except that of the carbohydrate diet. From the above results, the conclusions are obtained that the protein free diet makes the acethylcholine and cholinesterase activity lower, and the carbohydrate free diet, acethylcholine lower.