We studied the influence of glucose (G), insulin (I) and potassium (K+) on vulnerability to ventricular fibrillation (VF) of the nonischemic canine myocardium. Vulnerability was assessed by determining VF and repetitive extrasystole (RE) thresholds with a single stimulus applied to the right ventricular endocardium during the vulnerable period. Electrical testing of the heart was performed before and after 1 and 2 hours of infusing G (10 mg/kg per min) and I (0.025 U/kg per min) with and without K+. Infusion of glucose, insulin, and potassium (GIK) in 11 dogs significantly increased both VF (22%) and RE (33%) thresholds within the 1st hour and only the VF threshold (64%) in the 2nd hour. No significant changes in serum K+ concentration occurred. Spontaneous termination of VF was observed in six dogs during GIK infusion. Glucose and insulin infusion increased both VF and RE thresholds within the 1st (17% and 19%) and 2nd hours (43% and 43%). This occurred despite substantial reductions in serum K+ concentration. Lowering serum K+ by hemodialysis in six dogs decreased both VF and RE thresholds within the 1st (33% and 32%) and 2nd hours (35% and 14%). Restoration of the serum K+ concentration by KC1 infusion while maintaining dialysis returned the thresholds to control values. In six dogs, insulin infusion during low K+ dialysis increased the VF and RE thresholds despite a further reduction in serum K+ concentration. We concluded that insulin exerts a protective effect against vulnerability to VF in the normal canine heart. This salutary action is most marked when the drug's hypokalemic effect is prevented by concomitant K+ infusion.