When frog sartorius muscles recover from Na enrichment in the presence of external K, net K entry into the fibers occurs by both passive move- ment and active inward transport via a K pump. Under normal conditions, it has not been possible to experimentally distinguish these processes. Frac- tionation into the flux components must be accomplished from inferences con- cerning the K conductance or permeability during a period of rapid Na ex- trusion. The best estimates indicate that 60-80 % of the K entry occurs via the K pump. In the presence of Ba ions, the membrane permeability to K is very much reduced. Under these conditions, Na-enriched muscles underwent a normal recovery in the presence of external K, and the amount of inward K movement due to the K pump rose to over 90% of the total K entry. The characteristics of the K pump studied by this means were: (a) essentially com- plete inhibition by 10 -4 M ouabain, (b) inhibition by (Na~o, (c) activation by (K)o according to a rectangular hyperbola in the absence of (Na)o, (d) linear activation by (Na~i over a wide range in concentration, (e) zero or unde- tectably low pumping rate as (Na)i ~ 0, (f) the number of Na ions actively transported per K ion actively transported is 1.4-1.7 normally and 1.1 in the presence of Ba.