The objective of this study was to evaluate the safety and efficacy of a purified porcine insulin zinc suspensiona for treating dogs with uncomplicated diabetes mellitus. Fifty-three dogs were treated for 60 days after an initial dose determination period. The means of the blood glucose concentrations during 12-hour glucose curves and the means of the blood glucose nadir concentrations during 12-hour glucose curves for all dogs were determined before beginning insulin therapy (time 0), at the end of the dose determination period (time 1), 30 days after time 1 (time 2), and 60 days after time 1 (time 3). Presence of polyuria, polydipsia, and ketonuria was determined at each time point. Adequacy of control of hyperglycemia was based on 12-hour blood glucose curves and improvement in clinical variables (results of physical examinations, historic information, polyuria, polydipsia, and ketonuria). Safety was evaluated by questionnaire, performance of physical examination, CBC, serum chemistry profile, and urinalysis. The means of the blood glucose concentrations during 12-hour glucose curves and the means of the blood glucose nadir concentrations during 12-hour glucose curves for all dogs at times 1, 2, and 3 were significantly lower compared with time 0 (P < .0001). There was a reduction in the proportion of dogs with polyuria, polydipsia, and ketonuria of 82, 86, and 80%, respectively. All of the dogs had adequate glycemic control at time 1, 66% at time 2, and 75% at time 3. At time 3, 66% of dogs required insulin injections q12h. Other than hypoglycemia, there were no important adverse effects of insulin administration. The insulina was safe and efficacious for reducing blood glucose and clinical signs in dogs with diabetes mellitus.