Twenty-four patients--5 normals, 5 obese subjects, 9 normal weight maturity-onset diabetics and 5 obese maturity-onset diabetics--were subjected to stimulation with the gastro-intestinal hormone secretin (GIH, Stockholm, Sweden). Secretin was administered by rapid intravenous injection 6 times every 30 min in doses increasing from 0.9-90 pmol/kg body weight. Radioimmunological insulin assay and blood glucose determination were carried out. The secretin-induced maximum insulin response rose in all groups with increasing doses. D50 (16 pmol/kg body weight) was identical in all 4 groups. Thus, the diabetic B cells exhibited the same sensitivity to secretin as the normals. Obese subjects already showed a significant insulin response after 0.9 pmol secretin/kg, whereas the response of normal weight subjects did not rise until the dose was 3.6 pmol/kg. This, combined with the normal D50, indicates an increased B cell mass in obese subjects.