The effects of the oximes 2-pyridine aldoxime methiodide (PAM), HI-6, HS-6, toxogonin and TMB-4 on the rate of carbamylation of membrane-bound bovine erythrocyte acety1cholinesterase were studied. The second-order rate constant of carbamylation (k;) and the first-order rateconstant of decarbamylation ( k 3) were calculated from the proportion of free acetylcholinesterase at equilibrium and the rate of approach to equilibrium. Twenty insecticidal carbamates plus physostigmine and pyridostigmine were studied. The oximes increased k i for several carbamates, with HI-6 causing an increase in the most number of cases (12) and PAM the least (3). HI-6 was also a potent accelerator of decarbamylation (increase in k 3) in all cases, whereas PAM caused a significant decrease in k 3 in 15 cases and a non-significant decrease in the other 7. Toxogonin and TMB-4 increased k 3 or had no significant effect. The results were generally consistent with a proposal in the literature that there is a correlation between increased k i and increased toxicity of the carbamate in the presence of an oxime.