1. 1. Groups of lean and obese LA/N-cp and obese Type II diabetic SHR/N-cp rats were fed semisynthetic diets with or without the α-glucosidase inhibitor acarbose (ACB, 100 mg/kg diet, p.o.) from 8 until 15 weeks of age, and measures of fasting serum total cholesterol (TC), insulin (INS), and hepatic HMG-CoA synthase activity determined at the end of the study. 2. 2. ACB was without marked effect on mean food intake in either strain or either phenotype, and resulted in less weight gain and decreased adipose mass in obese LA/N-cp rats. INS was greater in the obese than the lean phenotype of both strains, and ACB resulted in greater reductions in INS in obese LA/N-cp than in obese LA/N-cp rats. 3. 3. Serum TC concentrations were greater in the obese than in the lean phenotype of both strains, and ACB resulted in decreases in TC in both strains and in lower beta: alpha lipoprotein cholesterol ratios in obese LA/N-cp rats. Liver HMG Co-A synthase activity was greater in lean than obese rats and ACB resulted in normalization of enzyme activity in obese LA/N-cp but not SHR/N-cp rats. 4. 4. These results confirm the hypercholesterolemia which occurs in the obese phenotype of the corpulent rat strains, and indicates that ACB may bring about significant reductions in body weight and fatness, TC, and in improved beta:alpha lipoprotein ratios and HMG-CoA synthase activity in obesity. The failure to observe similar improvements in body weight and fatness and in cholesterogenic activity in the obese diabetic SHR/N-cp rats is consistent with the greater degree of insulin resistance which occurs in the obese phenotype that strain.