1. Unidirectional K+ (86Rb) influx in lamprey red blood cells was studied under different conditions. 2. The influx of 86Rb was markedly inhibited by 1 mM Ba2+ when cells were incubated in saline containing 4 mM K+. In K(+)-free media, the influx rate constant of 86Rb was lower, and 1 mM Ba2+ had no blocking effect. 3. Treatment of the red cells with 0.1 mM ouabain in the absence of external K+ resulted in the appearance of the component of 86Rb influx inhibited by 1 mM Ba2+, quinine, TEA or amiloride. 4. Similar results were obtained in red cells incubated in Na(+)-free media MgCl2-sucrose. 5. The results obtained provide evidence for the existence of K+ channels in the red cell membrane of the lamprey. Under physiological conditions (in the presence of 4 mM K+) the total rate constant for the 86Rb influx in erythrocytes was about 1.9/hr, including ouabain-sensitive (0.6/hr), Ba(2+)-sensitive (1.1/hr) and residual (0.2/hr) components.
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