The reduction of ubiquinone (Q) in electron transport particles from beef heart, caused by succinate in the presence of the respiratory inhibitors Antimycin A and eyanide, appears to follow mixed zero- and first-order kinetics of the type expected for a simple enzymic reaction. The zero-order rate for Q reduction is the same in the presence of Antimycin A or cyanide, and is the same as that for cytochrome c reduction in the presence of cyanide and cytochrome b reduction in the presence of Antimycin A for equal numbers of electrons transported. The half-time for Q reduction is at least four times greater than that for the cytochromes under comparable conditions. A simple, approximate method was used to calculate K m for Q in the reduction reaction, assuming that the reaction follows simple Michaelis-Menten kinetics and that the Q is “dissolved” and uniformly distributed in the lipid phase of the particles. The calculated value approximates 1 mm.