The systemic insecticide methamidophos, MeO(MeS)P(O)NH2, is a very weak inhibitor of acetylcholinesterase (AChE) in vitro relative to in vivo suggesting bioactivation. This hypothesis is supported by finding that brain AChE inhibition and poisoning signs from methamidophos are greatly delayed in mice and houseflies pretreated with oxidase inhibitors in an order for effectiveness of methimazole > N-benzylimidazole >> piperonyl butoxide. In contrast, the order for delaying parathion-induced AChE inhibition and toxicity is N-benzylimidazole >> piperonyl butoxide or methimazole, suggesting that different oxidases are involved in methamidophos and parathion activation. N-Hydroxylation is examined here as an alternative to the controversial S-oxidation proposed earlier for methamidophos activation. N-Hydroxymethamidophos [MeO(MeS)P(O)NHOH], synthesized by coupling MeO(MeS)P(O)Cl and Me3SiNHOSiMe3 followed by desilylation, is unstable at pH 7.4 (t1/2 = 10 min at 37 degrees C) with decomposition by two distinct and novel mechanisms. The first mechanism (A) is N-->O rearrangement to MeO(MeS)P(O)ONH2 and then hydrolysis to MeO(MeS)P(O)OH, a sequence also established in the analogous series of (MeO)2P(O)NHOH-->(MeO)2P(O)ONH2-->(MeO)2P(O)OH. The second mechanism (B) is proposed to involve tautomerism to the phosphimino form [MeO(MeS)P(OH)=NOH] that eliminates MeSH forming a metaphosphate analogue [MeOP(O)=NOH] trapped by water to give MeO(HO)P(O)NHOH that undergoes the N-->O rearrangement as above and hydrolysis to MeOP(O)(OH)2. As a metaphosphate analogue, the metaphosphorimidate generated from MeO(MeS)P(O)NHOH in aqueous ethanol yields MeOP(O)(OH)2 and MeO(EtO)P(O)OH in the same ratio as the solvents on a molar basis. Reactions of the N- and O-methyl derivatives of MeO(MeS)P(O)NHOH and (MeO)2P(O)NHOH are consistent with proposed mechanisms A and B. N-Hydroxymethamidophos is less potent than methamidophos as an AChE inhibitor and toxicant possibly associated with its rapid hydrolysis. Bioactivation of methamidophos via a metaphosphate analogue would directly yield a phosphorylated and aged AChE resistant to reactivating agents, an intriguing hypothesis worthy of further consideration.