We studied the influence of extrusion cooking of cereal brans (defatted rice bran, D.R.B.; rye bran, R.B.; oat bran, O.B.) on their properties and physiological action in rats. The level of dietary fiber (DF) in each diet was adjusted to 5% as total DF. The animals were fed on one of the diets for 23 days. A smaller particle size (Ext. D.R.B.) and damaged particle surface (Ext. D.R.B. and Ext. O.B.) were observed in some extruded brans. The water-holding capacity of the brans was increased by extrusion, but the fecal water-holding capacity was not affected. Cecal weight and the length of the small intestine were significantly decreased in the rats fed the extruded products. Gastrointestinal transit time in the rats fed the extruded products did not differ from that in rats fed the non-extruded products. The apparent rates of digestion and absorption of protein were increased, but these of fat were decreased by extrusion. Apparent absorption rates of Ca, P, Mg and Fe were lower in the rats given the extruded products than in those given the non-extruded products. The fecal excretion rate of DF was decreased significantly in the rats given the extruded products, being especially evident for Ext. O.B.