Verapamil exacerbates the increase in serum potassium after a large-dose potassium infusion or after the IV administration of succinylcholine. We conducted a study in 12 canines conditioned for 30 days acting as their own controls. The canines had chronic tracheotomies and carotid loops performed 2 wk before the experiment. Control canines were given 1 mg/kg succinylcholine at Time 0. Blood samples were analyzed for potassium at 0, 1, 3, 5, 10, 15, 30, and 60 min. One week later, the dogs received 0.15 mg/kg of either verapamil or diltiazem, followed by a 5.6-microg x kg(-1) x min(-1) 10-min infusion of the same drug. The animals were then given a bolus dose of succinylcholine 1 mg/kg, and the blood potassium was analyzed as before. There was no significant difference in the potassium concentration before the succinylcholine injection (Time 0) between the study groups. The canines pretreated with verapamil had a significantly greater increase (24% +/- 8%) in potassium concentration than the control canines (14% +/- 6%) 15 min after succinylcholine administration. There was no difference between the potassium concentrations of the diltiazem-pretreated canines and the control group at any time point. Therefore, diltiazem pretreatment does not significantly influence potassium regulation after a succinylcholine injection, whereas verapamil pretreatment has measurable hyperkalemic effects. CONCLUSIONS Succinylcholine is a drug that causes blood potassium to increase. Potassium influences heart rhythm. Verapamil and diltiazem are drugs used for angina heart pain. We used dogs to determine the effect of verapamil or diltiazem on the blood's potassium after an injection of succinylcholine and found that verapamil had the greatest effect.