In vitro, γ-guanidinobutyramide (HL523) stimulates in muscle glucose uptake, glycogenosis, glycolysis and glucose oxidation; in adipose tissue glucose oxidation; and insulin secretion in pieces of pancreatic tissue incubated at physiological glucose concentration. These metabolic effects of HL523 can account for its hypoglycemic action in vivo. 1 This work was supported in part by a grant from the Fonds National de la Recherche Scientifique (W.J.M.), and by the association contract Euratom-Universities of Pisa and Brussels (026-63-04 BIAC).