The insulin-releasing ability of some neutral and dibasic amino acids was studied in an in vitro system using micro dissected pancreatic islets with a high proportion of insulin-producing β-cells. The neutral amino acids L-alanine and α-aminoisobutyric acid had no substantial effects on basal and glucose stimulated insulin release. In contrast, 5–20 mM of the dibasic amino acid L-arginine gradually enhanced the insulin release induced by 10 mM glucose. A critical glucose concentration must be reached before L-arginine can act as an insulin secretagogue. N-methyl-L-arginine lacked the stimulatory ability. Co-transport of neutral amino acids and sodium does not elicit insulin release in adult mouse β-cells. There may, however, be a close relationship between the transport site for L-arginine and a receptor triggering insulin release.