Opioid-active substances have been isolated from bovine beta-casein peptone (beta-casomorphin). Since the ingestion of beta-casomorphin-containing foodstuff elicits an increase in postprandial insulin release, the present study was designed to determine the effect of iv infused beta-casomorphins on insulin release. The infusion of beta-casomorphin-7, -5, -4, and -3 did not alter basal insulin secretion. During prestimulation of insulin release with amino acids and glucose the infusion of beta-casomorphin-7, -5, -4, and -3 at a dose of 1 nmol/ kg . h augmented insulin release, whereas at a concentration of 100 nmol/kg . h no further stimulatory effect was observed except for the infusion of beta-casomorphin-4. In comparison, the infusion of morphine elicited a potentiation of insulin release at the lower dose, whereas no effect was observed at the higher infusion rate. Leucine-enkephalin had no effect at the lower dose but elicited an inhibitory effect at the higher rate. The infusion of opiate-active and inactive analogs of beta-casomorphin-4 resulted in a loss of its stimulatory effect, indicating that the full tetrapeptide structure of the molecule is important for its stimulatory activity on beta-cell secretion. All stimulatory and inhibitory effects of the opioid-active substances were blocked by the specific opiate-receptor antagonist naloxone. Additionally, the present data support the concept that beta-cell secretion is stimulated by mu-receptor activation and inhibited by delta-receptor activation. The K-receptor antagonist ethyl-ketocyclazocine did not alter insulin secretion. The fact that iv administered beta-casomorphins stimulate insulin release raises the possibility that ingested food-derived opioid-active substances modulate pancreatic endocrine function also after their absorption, provided the doses employed in the present study reflect physiological concentrations of circulating beta-casomorphins.