Force-velocity relations, rate of ATP turnover (JATP), and phosphorylation of the 20,000 D myosin light chains (LC20) were measured in chemically skinned guinea pig Taenia coli. Relative LC20 phosphorylation at 3.2 mM MgATP was 17% in relaxed tissues at pCa 9, and increased with force at increasing [Ca2+] to a maximum of 67% at pCa 4.5. Force at pCa 4.5 was dependent on the MgATP concentration with a half-maximal response at about 0.1 mM. At 0.1 mM MgATP LC20 phosphorylation at pCa 4.5 was 38%. Both JATP and the maximal shortening velocity (Vmax) were reduced in 0.1 mM MgATP, to 32% and 43%, respectively, of their values at 3.2 mM MgATP. Low-MgATP thus inhibits both LC20 phosphorylation and the extent and rate of cross-bridge interaction. High levels of LC20 phosphorylation, independent of Ca2+ and MgATP concentrations, were obtained by treatment with ATP-gamma-S. Maximal force at 3.2 mM MgATP after LC20 thiophosphorylation was unchanged, whereas halfmaximal force occurred at 0.065 mM MgATP after thiophosphorylation, compared to 0.13 mM after activation by Ca2+. The contraction in thiophosphorylated preparations at low-MgATP (0.1 mM) was associated with submaximal Vmax (60%) and JATP (27%). The results show that LC20 phosphorylation is correlated with the degree of force development in the Ca2+ activated contraction, both when Ca2+ and MgATP concentrations are varied. The reduced force and rate of crossbridge turnover in low MgATP are however primarily mediated by an influence of MgATP on the cross-bridge cycle, which is separate from the effect on LC20 phosphorylation.