To evaluate whether dietary fats of different degrees of unsaturation alter glucose and very low density lipoprotein-cholesterol (VLDL-CH) homeostasis, normal and alloxan-diabetic pigs were fed diets containing either beef tallow or soy oil as the primary source of fat for 6 weeks. After intra-arterial and oral doses of glucose, pigs fed soy oil had similar glucose and greater insulin concentrations in plasma when compared with pigs fed beef tallow. Beef tallow-fed pigs additionally were 40% more glucose effective than were soy oil-fed pigs. Concentrations of triglyceride, free fatty acid, and gastric inhibitory polypeptide in plasma were measured after consumption of a meal and did not vary between dietary treatments in normal pigs. Diabetic pigs fed soy oil, however, had greater concentrations of gastric inhibitory polypeptide after a meal than did those fed beef tallow;Disappearance of injected autologous ('14)C-VLDL-CH was analyzed in pigs using a two-pool model. Diabetes resulted in a twofold increase in half-lives and a 60-fold increase in pool sizes of the primary and secondary components of VLDL-CH disappearance when compared with those of normal pigs. In normal pigs, feeding beef tallow resulted in longer half-lives of both components of VLDL-CH disappearance and no effect in pool size of both components of VLDL-CH disappearance than did feeding soy oil. In comparison, diabetic pigs fed beef tallow had a similar half-life of the primary component, a twofold shorter half-life of the secondary component, and threefold larger pool size of the primary component, and a similar pool size of the secondary component of VLDL-CH disappearance than did diabetic pigs fed soy oil. Accretion of plasma cholesterol in liver, heart, aorta, skeletal muscle, and adipose tissue was 1.5 to 2 times greater in diabetic than in normal pigs, and net transfer rate of plasma cholesterol was greater only in liver of diabetic when compared with normal pigs. Thus, dietary fat seems to play an important role in regulation of glucose and VLDL-CH homeostasis in normal and diabetic animals.