The effect of temperature on the kinetics of the deacetylation of diamorphine and 6-monoacetylmorphine was studied in human plasma. Diamorphine was rapidly and quantitatively degraded to 6-monoacetylmorphine with initial half-lives of 354, 18 and 3 min at temperatures of 4, 25 and 37 degrees C, respectively. Further deacetylation to morphine was not detected. In aqueous solution, diamorphine was quantitatively degraded to give 6-monoacetylmorphine as the major product and morphine as a minor product, the rate of deacetylation being dependent on temperature and pH. At pH 4.0 and 5.6 diamorphine had a half-life of greater than 14 days at all temperatures but at alkaline pH diamorphine was rapidly deacetylated. The rate of deacetylation of 6-monoacetylmorphine was consistently slower than that of diamorphine under identical conditions of pH and temperature. A method is described for the rapid stabilization and subsequent assay of diamorphine in plasma which will prevent errors in estimation of the drug due to unwanted hydrolysis.