The effect of DC electric field strength on in vitro actomyosin motility was examined. Rabbit skeletal muscle heavy meromyosin (HMM) was adsorbed to nitrocellulose-coated glass, and the myosin driven movement of fluorescently labeled actin filaments was recorded in the presence of 0 to 9000 V m/sup -1/ applied DC voltage. The applied electric field resulted in increased filament velocity and oriented actin movement, with leading heads of filaments directed towards the positive electrode. Velocity (v) was found to increase moderately with electric field strength at applied fields up to /spl sim/4500 V m/sup -1/, and then increased more rapidly and irregularly at higher field strengths up to 9000 V m/sup -1/. The electrophoretic effect caused up to 70% of actin motion to be oriented within 30 degrees of the positive electrode, with the largest effect observed using an applied field of 6000 V m/sup -1/. Higher electric field strengths were found to cause extensive filament breakage.