1. A study is made of the effect of adenosine triphosphate (ATP) upon the viscosity of solutions of actomyosin in 0.5 M KCl.
2. The observed effects are discussed in terms of an initial drop of the viscosity (viscosity response) and its subsequent slow reversal (recovery effect). The latter is ascribed to a decrease in the ATP concentration through enzymatic hydrolysis.
3. The recovery effect is inhibited by Mg, activated by Ca, in accordance with the effect of these ions on the activity of myosin-ATPase.
4. The viscosity response is not inhibited, probably promoted by Mg. It is not promoted, probably inhibited by Ca.
5. The viscosity response is induced not only by ATP, but to a certain extent also by inosinetriphosphate, inorganic triphosphate, and inorganic pyrophosphate, not by adenosine diphosphate or monophosphate.
6. The viscosity response could be obtained with enzymatically inactive myosin.
7. It is concluded that the effect of ATP upon myosin does not depend on its enzymatic hydrolysis.