Hydrogen sulfide is synthesized endogenously in mammals and has been shown to have both physiological and pathological functions. So far there has been little agreement as to the actual levels of endogenous sulfide under physiological or pathological conditions; this is partly due to the complexity involved in measuring free sulfides due to H 2 S volatility, oxidation, reactivity and the presence of bound labile sulfur in tissues. In this report we describe a method of measuring free tissue sulfides using a zinc sulfide precipitation and wash method. It is an indirect method that measures the sulfide difference between samples prepared at pH 9 and pH 6, assuming that at pH 9 free sulfides would be retained in solution, while at pH 6 free sulfides would volatilize during sample preparation. Using this approach we were able to measure appreciable amounts of free sulfides in mouse: lung, pancreas, liver and kidney at 0.036 + 0.006, 0.082 + 0.009, 0.215 + 0.016 and 0.323 + 0.031 nmole per mg of tissue respectively (n = 6).