The effect of K+ on the respiratory control of rat liver mitochondria partially depleted of K+ has been studied. Although the P O ratios are not ostensibly affected by K+, the State 3/State 4 respiratory ratio is considerably enhanced by the addition of the cation to the incubation mixture with NAD dependent substrates. The results are suggestive that K+ is not required for the formation of ATP during coupled electron transport, but that the respiratory control of mitochondria depends on the K+ content of the mitochondria. © 1969.