Chronic oxytetracycline treatment was found to alter the diabetic status of the spontaneously diabetic rat (BB rat). The treatment led to lowered plasma glucose levels in the fed as well as in the fasted state. These results indicate that the oxytetracycline treatment was effective in lowering the insulin requirements as well as in improving the handling of glucose. The effects of the drug are not secondary to the decreased food intake as a food restricted control group did not show the improvements in glycemia or glucose and insulin tolerance. These results are a further indication that oxytetracycline enhances the response of peripheral tissues to insulin and thus favors better control of glycemia.