The aim of this study was to investigate the effect of a synthetic fifteen-residue peptide, Gly-Thr-Pro-Gly-Pro-Gln-Gly-Ile-Ala-Gly-Gln-Arg-Gly-Val-Val(P-15),on islet cell insulin secretion and insulin gene expression.Wistar rat islets,isolated by collagenase and trypsin digestion,were exposed to 0,0. 72,7.2,36?mol/L P-15 with 16.7mmol/L glucose.Insulin release increased approximately twofold in re- sponse to the concentration of 7.2?mol/L P-15 and Northern-blot analysis of islet mRNA with a rat pre- proinsulin cDNA probe showed a concomitant increase in mRNA level as compared with levels observed in islets treated with control.It is concluded that P-15 not only can stimulate the release of insulin but also in- crease the preproinsulin mRNA expression of islet cells.