The C-terminal eight-amino acid derivative of CCK, sulfated on the tyrosine residue (CCK8S), stimulated a dose-dependent biphasic pattern of insulin secretion from isolated perifused islets in the presence of 7 mm glucose. It was without any effect if glucose were absent from the medium or maintained at 4 mm. The response to CCK8S was readily reversible and dependent on the presence of extracellular calcium. While CCK8S did not increase glucose usage rates above those noted with 7 mm glucose alone, inclusion of the metabolic inhibitor 2-deoxyglucose lowered glucose usage rates to values obtained with 3–5 mm glucose and abolished the influence of CCK8S on insulin output. Removal of the metabolic inhibitor restored the secretory response. N-Acetylglucosamine (15 mm) or glyceraldehyde (2.5 mm) substituted for glucose and permitted CCK8S to evoke secretion. The nonsulfated eight-amino acid derivative of CCK, CCK8, provoked insulin secretion in the presence of 7 mm glucose, but only at 10–100 times greater levels than CCK8S. CCK4 (1 μm) did not influence insulin output in the presence of 7 mm glucose. On an equimolar basis, CCK8S was significantly more effective than gastric inhibiting polypeptide in augmenting insulin output. The results support a role for CCK8S in the regulation of insulin levels in vivo. (Endocrinology119: 616–621, 1986)