Fifth instar larvae ofBombyx mori were administered with additional Na+ ion orally. The concentration of Na+ increased and that of K+ decreased significantly in the haemolymph within 24 h. There was no marked difference in the ionic concentration of the midgut cell while K+ increased in the gut content. The protease activity increased significantly in gut lumen with a resultant increase in amino acid concentration, which showed a decrease in gut tissue and haemolymph. The protein content did not show significant change in gut lumen, while in gut tissue and haemolymph it decreased markedly. The results are discussed in relation to the Na+-linked nutrient transport mechanism in the gut epithelium ofBombyx mori.