A simple method is described for determining cysteinesulfinic acid decarboxylase activity in rat tissue. Enzyme preparations from the liver, kidney and brain were incubated with cysteinesulfinic acid substrate in the presence of pyridoxal 5-phosphate. The enzyme product, hypotaurine, was derivatized with o-phthalaldehyde and separated by reversed-phase high-performance liquid chromatography (Capsel Pack AG 120A C18 column) using a mobile phase of acetonitrile-water (20:80, v/v) containing 50 mM sodium phosphate buffer (pH 6.0) and detected using a fluorometer (excitation at 360 nm and emission at 455 nm). The method described is reproducible and sensitive enough to determine the activity of cysteinesulfinic acid decarboxylase activity in the liver, kidney and brain. This assay was subsequently used to evaluate the effect of dietary proteins whose sulfur amino acid contents differ. Consistent with reported data, compared to casein and whole egg protein, a dietary protein low in sulfur amino acid (soybean protein) increased cysteinesulfinic acid decarboxylase activity in the liver and kidney. This method is therefore applicable to studies on the dietary regulation of cysteinesulfinic acid decarboxylase in rat tissue.