The microsomal fraction from the electric organ of the eel Electrophorus electricus catalyzes the hydrolysis of adenosine triphosphate in the presence of Mg(++), Na(+), and K(+). The same preparation catalyzes a Mg(++)-dependent transphosphorylation between adenosine triphosphate and adenosine diphosphate. Both of these reactions are inhibited after treatment of the microsomes with N-ethyl maleimide. However, the addition of Na(+) reactivates the transphosphorylation, and the rate becomes more rapid than that of the original. This new Na(+)-sensitive exchange reaction is believed to be a component of the hydrolytic reaction.