In the presence of adenosine 5'-triphosphate (ATP) and 1-10 mM MgCl2, the relative viscosity (eta rel) of dephosphorylated gizzard myosin is reduced markedly over a range of KCl from 0.35 to 0.15 M. Sedimentation patterns show that the decrease in eta rel is due to the conversion of the 6S to 10S forms of myosin. Under similar conditions, eta rel of phosphorylated myosin is not altered, and at 0.2 M KCl, the 10S form is not observed. In 1 and 2 mM MgCl2 and less than 0.2 M KCl, 10S can be formed from both phosphorylated myosin plus ATP and dephosphorylated myosin minus ATP. In the presence of ethylenediaminetetraacetic acid (EDTA), the decrease of eta rel and corresponding change in sedimentation pattern are independent of ATP and show only a dependence on KCl. Therefore, ATP and dephosphorylation are not obligatory for the 6S to 10S transition. In all instances, the 6S-10S transition of monomeric myosin is paralleled by an alteration of adenosine-5'-triphosphatase (ATPase) activity; i.e., the KCl dependence of the two processes is the same. Transition from 6S to 10S causes a decrease in Mg2+-and Ca2+-ATPase activity of myosin and an increase in K+-EDTA-ATPase activity. The relationship between myosin shape and the ATP dependence of Mg2+-ATPase activity also is consistent with this generalization. The phosphorylation dependence of the viscosity transition from 6S to 10S is not linear, and phosphorylation of both heads is required for the complete transition. In contrast, the ATP dependence of the transition is linear, and the binding of 2 mol of ATP/myosin is required for maximum effect.(ABSTRACT TRUNCATED AT 250 WORDS)