Addition of increasing concentrations of K+ to yeast mitochondria in the presence of 0 to 400 microM phosphate and 200 microM Mg2+ led to uncoupled respiration and decreased protonmotive force (deltaP):at 0 K+ deltaP = 213 mV, negative inside, where deltapsi = 180 mV and deltapH = 33 mV, while at 20 mM K+ deltaP = 28 mV, where deltapsi = 16 mV and deltapH = 12 mV. In contrast, the synthesis of ATP resulted in smaller values for the Km and the Vmax in 400 microM Pi and increasing ADP: in 0 K+, Km = 18.6 microM and Vmax = 75.4 nmol (min x mg protein)-1, while in 20 mM K+, Km = 5.2 microM and Vmax = 46.0 nmol (min x mg protein)-1, i.e., when K+ depleted most of the deltaP, and at ADP concentrations below the Km, the rate of ATP synthesis was essentially the same as in the absence of K+. At saturating ADP, the rate of ATP synthesis in the presence of K+ was about 60% of the rate observed without K+. The synthesis of ATP by yeast mitochondria was inhibited by oligomycin or uncouplers. K+ had no effects on rat liver mitochondria. Adenylate kinase activity was much smaller in yeast mitochondria than in rat liver mitochondria and thus did not account for the synthesis of ATP observed in the presence of K+. The effects of K+ on the deltaP of yeast mitochondria were prevented by increasing concentrations of phosphate (1 to 4 mM). At 4 mM phosphate, the deltaP was always above 200 mV and the kinetics of ATP synthesis were as follows: 0 K+ Km = 10.0 microM and Vmax = 88.3 nmol (min x mg protein)-1. At 20 mM K+, Km = 7.4 microM and Vmax = 133 nmol (min x mg protein)-1.