Vegetarians and other persons with high fiber intakes have lower average blood pressures than persons with low fiber intakes. We assessed blood pressures in 12 nonobese, insulin-treated, diabetic men on control diets and high carbohydrate and fiber (HCF) diets. The control diets provided 20 g/d of plant fiber, whereas the HCF diets provided 65 g/d. The insulin doses required to maintain satisfactory glycemic control were 73% lower with HCF diets than with control diets. Average blood pressures were 10% lower (p less than 0.01) with HCF diets than with control diets. In a study of six additional men, fecal water excretion was twofold greater with HCF diets than with control diets. Alterations in gastrointestinal function, nutrient absorption rates, and secretion of gastrointestinal and pancreatic hormones may influence blood pressure. The possible antinatriuretic effects of insulin may influence blood pressure responses to fiber intake. Further studies are needed to assess critically the effects of fiber intake on blood pressure.