1. The distribution of azide added to suspensions of bakers' yeast was studied under various conditions. The recovery of azide was estimated in the volume of water into which low concentrations of electrolytes can readily diffuse (anion space). Considerable azide disappeared from this anion space.
2. The incomplete recovery of azide in the anion space is due to its uptake by the cells. This uptake occurs against a concentration gradient at 0°C., and is attributed to binding of azide by cell constituents.
3. Confirmatory evidence is presented that one such constituent is the K carrier in the cell membrane. The azide inhibition of K transport is not mediated by inhibition of cytochrome oxidase in the mitochondria.
4. From the amount of combined azide and the experimentally determined dissociation constant of the K carrier-inhibitor complex, the maximum value for the concentration of this carrier is calculated as 0.1 µM/gm. yeast.
5. The addition of glucose and PO4 causes a secondary K uptake which is not azide-sensitive and is clearly distinct from the primary, azide-sensitive mechanism.
6. The existence of a separate carrier responsible for Na extrusion is reconsidered. It is concluded that present evidence does not necessitate the assumption that such a carrier is active in yeast.