The effect of quinidine on unidirectional fluxes of Na and K in isolated rabbit atria has been investigated and correlated with known effects of the drug on the electrical and mechanical properties of the heart. Quinidine caused a rapid inhibition of Na entry, which secondarily depressed K efflux. In addition, the drug depressed membrane permeability to K and caused a marked inhibition of active transport, both of which were relatively slow in onset.