An inwardly rectifying K+-selective conductance was identified in lamprey erythrocytes using whole-cell patch-clamp. The cation selectivity sequence of the whole-cell conductance was K+ > Rb+ greater than or equal to Cs+ > Na+ greater than or equal to Li+ as determined from current reversal potentials. Virtually no anion conductance was found. There was a great cell-to-cell variation in the magnitude of inward current carried by K+ (2.97 +/- 1.59 nS, N = 16). Inward current was blocked by 2 mM external Ba2+. Amiloride (1 mM) and tetrapentylammonium bromide (TPeA; 0.1-1 mM) were less effective blocking agents, and Cs+ (5-10 mM) produced voltage-dependent block of inward currents. The whole-cell K+ conductance was not dependent on Ca2+ (1-10 mu M) or ATP and GTP (1 mM) in the pipette solution, but appeared to be dependent on external K+, as the cell was depolarized when the K+ concentration was decreased below 10 mM in a bath containing Na+ as the major cation (and with K+ as the major cation in the pipette).