The pharmacokinetics and pharmacodynamics of porcine insulin zinc suspension were studied in eight dogs with spontaneous diabetes mellitus and a lack of endogenous insulin, demonstrated by the lack of insulin secretion after an injection of glucagon and confirmed by measurement of the concentration of C-peptide during the pharmacokinetic assessments. After daily subcutaneous injections of porcine insulin zinc suspension, concentrations of insulin and glucose that differed significantly from baseline were identified using the 90 per cent range of differences. In all the dogs there was an initial peak concentration of insulin approximately three hours after the injection and a second peak after approximately nine hours in all but one of them. The serum insulin concentration remained above baseline for a mean of 15.5 hours. The concentration of glucose was reduced significantly after the injections in all except one dog, which was normoglycaemic at baseline. The action of insulin began after approximately three hours and the minimum glucose concentration occurred after approximately eight hours. The median duration of action of the insulin was approximately 14 hours, with a range from 10 to more than 24 hours.