4-Acetamido-4′-isothiocyanostilbene-2,2′-disulphonic acid (SITS), an amino-reacting probe of plasma membranes, stimulated the release of insulin from micro-dissected pancreatic islets of ob/ob-mice. This effect of SITS was inhibited by adrenaline or by calcium deficiency. SITS did not inhibit the insulin-releasing action of glucose or leucine but rather potentiated the effect of glucose. In contrast, SITS markedly depressed the insulin secretory response to chloromercuribenzene-p-sulphonic acid. It is suggested that by reacting with the plasma membranes SITS may induce secretagogic ionic fluxes in the β-cells. In addition, SITS apparently inhibits the secretagogic recognition of chloromercuribenzene-p-sulphonic acid, presumably by preventing the organic mercurial from reacting with certain membrane thiol groups.