The effect of steroids on the uptake and release of K+ by capillaries isolated from rat brain was investigated. 86Rb' uptake was used as a transport analog of K+. The uptake of 86Rb' by the capillaries was markedly inhibited by ouabain. The ouabain sensitive fraction of 86Rb' uptake was inhibited by corticosterone with an 150 of 8 X 10m4M. Inhibition was immediate in onset and rapidly reversible after removal of the steroid. Corticosterone did not alter the affinity of the transport carrier for K+ or the passive efflw of '6Rb' from the capillaries. Other steroids inhibited ouabain sensitive %Rb+ uptake in relation to their lipid solubihty. These features suggest a direct membrane action for high dose steroid therapy, possibly on Na+,K+-ATFase, rather than a nuclear mediated change in cell