Objective: To investigate the reactivating of diacetyl monoxime (DAM) on mouse small intestine carboxylesterase (CaE) inhibited by dichlorvos(DDVP) in vitro and in vivo. Methods: A p nitrophenol valerate microassay method for CaE activity was used to determine the reactivating effect of DAM on mouse small intestine CaE inhibited by DDVP in vitro and in vivo. Results and Conclusion: The reactivation percentages of DDVP inhibited mouse small intestine CaE induced by DAM at 1,0.5 and 0.1 mmol/L in vitro were 84.8%,59.3% and 32.4%, respectively. It did not inhibit the normal small intestine CaE. Administration of DAM po, im or ip at 100 mg/kg body weight did not exhibit significant antitoxic effect on mice intoxicated by DDVP po at 100 mg/kg body weight and also did not induce significant reactivation of small intestine CaE in mice intoxicated by DDVP po at 50 mg/kg body weight (P>0.05). Furthermore, DAM po and ip at 100 mg/kg body weight could decrease small intestine CaE activity significantly in normal mice (P<0.05). These results suggested that administration of DAM po, im or ip does not have significant antitoxic effect on mice intoxicated by DDVP po. The inhibitory effect of DAM itself on mouse small intestine CaE activity may be related to its metabolism in liver and intestine.