The phosphatase inhibitor okadaic acid (OA) was used to study the relationship between [Ca2+], rates of phosphorylation/dephosphorylation and the mechanical properties of smooth muscle fibres. Force/velocity relationships were determined with the isotonic quick release technique in chemically skinned guinea-pig taenia coli muscles at 22 degrees C. In the maximally thiophosphorylated muscle neither OA (10 microM) nor Ca2+ (increase from pCa 9.0 to pCa 4.5) influenced the force-velocity relationship. When the degree of activation was altered by varying [Ca2+] in the presence of 0.5 microM calmodulin, both force and the maximal shortening velocity (Vmax) were altered. At pCa 5.75, at which force was about 35% of the maximal at pCa 4.5, Vmax was 55% of the maximal value. When OA was introduced into fibres at pCa 6.0, force was increased from less than 5% to 100% of the maximal force obtained in pCa 4.5. The relationship between the degree of myosin light chain phosphorylation and force was similar in the two types of activation; varied [OA] at constant [Ca2+] and at varied [Ca2+]. The relation between force and Vmax when the degree of activation was altered with OA was almost identical to that obtained with varied [Ca2+]. The results show that Ca2+ and OA do not influence force or Vmax in the maximally phosphorylated state and suggest that the level of myosin light chain phosphorylation is the major factor determining Vmax. The finding that the relationship between force and Vmax was similar when activation was altered with OA and Ca2+ suggests, however, that alterations in the absolute rates of phosphorylation and dephosphorylation at a constant phosphorylation level do not influence the mechanical properties of the skinned smooth muscle fibres.