1. 1. The influence of the two neutral aminoacids l-alanine and glycine on the intestinal transepithelial potential difference (pd) in Bombyx mori midgut in vitro has been tested. 2. 2. While glycine causes a more rapid drop of the pd, l-alanine gives rise to a relevant pd increase when added both to the mucosal or to the haemolymphatic side of the tissue. The presence of sodium is not necessary to obtain the pd rise. 3. 3. A negative correlation has been found between pd values before l-alanine addition and the subsequent pd increments. 4. 4. Since pyruvic acid, metabolic product of l-alanine, mimics l-alanine effect, it is suggested that l-alanine could provide metabolic supply for the K pump.