Species-related differences in sensitivity to acute intoxication by anticholinesterase compounds have been attributed, in large part, to differences in the kinetics of inhibition of acetylcholinesterase (AChE) in vitro. Since inhibition of AChE is also influenced by the stability of the phosphorylated enzyme complex, it was of interest to compare the rates at which the inhibited enzyme from different species subsequently either reactivates or ages. Brain AChE from rats, mice, fathead minnows, or rainbow trout was preincubated with an IC90 concentration of either paraoxon or malaoxon. The first-order rate constants for both the reactivation and aging of paraoxon-inhibited AChE from rats and mice were significantly greater than those observed for either species of fish. Following malaoxon inhibition, however, rodent AChE reactived more rapidly but aged more slowly than did the enzyme from minnows. Therefore, the data suggest that compared to rodents, intermittent or continuous exposure of fish to sublethal concentrations of anticholinesterase compounds is more likely to result in a cumulative toxicity owing to the relative irreversibility of AChE inhibition.