The kinetic consequences of acetylcholinesterase peripheral site occupation by metal ions were examined using three substrates; acetylthiocholine, p-nitrophenylacetate, and 7-(dimethylcarbamoyloxy)-N-methylquinolinium iodide. Two classes of metal ion effects were noted: activation by a group including Mg2+, Ca2+, Mn2+, and Na+, and inactivation by a second group which to date includes Zn2+, Cd2+, Hg2+, Ni2+, Cu2+, and Pb2+. Activation is demonstrable only in solutions of low ionic strength whereas inactivation can be readily observed in solutions of both low and high ionic strength. Activation appears to be due to a combination of metal ion binding and ionic strength effects and involves binding to peripheral sites which are distinct from those which bind organic cationic activators such as gallamine, propidium, and 7-(dimethylcarbamoyloxy)-N-methylquinolinium. The principal activating effect is on the deacylation phase of the enzyme-substrate reaction. Inactivators effect a slow conversion of the enzyme to an unreactive form. The kinetics of inactivation are biphasic at low ionic strength but become essentially monophasic at high ionic strength. More than 80% of the enzyme activity can be recovered upon addition of EDTA provided the chelating agent is added immediately following completion of the inactivation process. Prolonged exposure to inactivators results in a progressive decrease in the amount of recoverable activity, Although peripheral ligand interactions may result in a variety of catalytic site conformations, the macroscopic properties can be accounted for in terms of three ligand-dependent states of the enzyme in which catalytic ability (actual or potential) is retained, and a fourth denatured state.