The effect on pregnant rats of individual amino acids added excessively to the diet or intermittently administered in excess directly with a stomach tube was examined. When methionine was excessively added to the diet at a 5% level, amino acid imbalance in plasma was induced and food intake decreased remarkably to approximately one-fifth of that of control rats. However, when administered directly into the stomach, food intake remained almost normal, except for excess of methionine, tryptophan or threonine, and an adverse effect of excess amino acid was not observed. However, in the case of a decrease in food intake, various adverse effects were observed. This was especially so for the group that was administered methionine and whose food intake was far below that of the control group: decrease in maternal body weight, delayed growth of products of conception, and further, significant decrease in brain and liver cells of the fetus were observed. In addition, the changes in nitrogen balance were well correlated with the changes in bodyweight in all groups. In the methionine group, in particular, nitrogen balance was negative throughout the period because of a decrease in food intake, and utilization of dietary nitrogen was inferior. The majority of the various lesions observed with excess administration of individual amino acids were not due to a direct effect of excess amino acid but were mainly caused by the remarkable decrease in food intake.