In order to clarify the mechanism of the adverse effects of dietary amaranth, trisodium 1-(4-sulfo-1-naphthylazo)-2-naphthyl-3,6 disulfonic acid, the effects of amaranth in vitro and in a jejunum perfusion in vivo on intestinal sucrase were investigated in rats. The inhibitory effect of amaranth in vitro on the sucrase activity was not detected even at the concentration of 1%, whereas the remarkable release of intestinal sucrase from intestine was observed with the jejunum perfusion of Ringer bicarbonate solution (RBS) containing amaranth at the 1% level. On the other hand, the perfusion of RBS containing tris(hydroxymethyl)-aminomethane, a strong inhibitor of intestinal disaccharidase activities, did not produce the release of intestinal alkaline phosphatase. These findings suggest that the toxicity of dietary amaranth is due to the exfoliating or solubilizing effects of amaranth on the brush border membrane of the small intestine.