The effects of different soluble fibers on blood glucose and insulin responses to voluntary meals in the rat were investigated. Rats (165–180 g) were adapted for 2 weeks to a fiber-free control diet or fiber diets made by dilution of the fiber-free diet with the fiber. Fibers and concentrations used were carboxymethylcellulose (1.25, 2.5, and 5%), guar gum, oat β-glucan (2.5, 5, and 7.5%), and mustard mucilage (5, 10, and 15%) as the soluble fibers, and cellulose (20%) as the insoluble fiber. Meal challenges (0.75 g/100 g body weight offered for consumption within a period of 15 min) were made with the adaptation diet. Soluble fibers reduced food intake and growth with a concentration effect, while cellulose increased food intake. All fibers reduced dry matter and nitrogen digestibilities but had no effect on the protein efficiency ratio. Only carboxymethylcellulose (at 5%) reduced postprandial glycemia at 30 min (p < 0.05). Cellulose had no effect on plasma insulin which was markedly reduced with a concentration effect by all soluble fibers (except mustard mucilage) in the following decreasing order: carboxymethylcellulose, guar gum, β-glucan, and mustard mucilage. The effect of fibers seemed closely related to the viscosity of fiber solutions in the presence of the diet ingredients. The present findings demonstrate the positive effect of fibers on postprandial insulinemia but with only a slight effect on glycemia. They indicate that the physicochemical interaction of the fibers with other dietary ingredients is important.