Two methods for measurement of cyst(e)amine in physiological samples are described. One method involves reduction of disulfides present in the sample with tributylphosphine, reversed phase chromatography of thiols, and electrochemical detection of cysteamine and other thiols. The other method involves reduction of disulfides with dithiothreitol, derivatization of thiols with 7-diethylamino-3-(4′-maleimidylphenyl)-4-methylcoumarin, separation of these derivatives by reversed phase chromatography, and fluorometric detection of the thiol adducts. The endogenous concentration of cysteamine in rat liver was estimated to be less than 2.5 nmol/g. Cysteamine is produced in tissues postmortem; rapid sampling/freezing of tissues and rapid inactivation of enzymes during tissue preparation are essential for accurate measurement of endogenous cysteamine concentrations.