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1. The influence of low concentrations (5 X 10(-8) to 5 X 10(-7) M) of ouabain on the K gradient in sheep cardiac Purkinje fibres was observed by measuring changes in the reversal potential for a K specific current iK2, and by measuring total steady-state current-voltage relations. 2. Provided that the bathing solution K concentration, [K]o was not too low, these doses of ouabain were often observed to increase the K gradient, i.e. the reversal potential was shifted in a negative direction. 3. The change in the reversal potential and in the current-voltage relation could be mimicked by reducing the value of [K]o in the absence of ouabain. It is therefore suggested that ouabain may stimulate the Na+-K+ exchange pump and so reduce the K concentration, [K]e, in the clefts of the preparation. 4. At sufficiently low values of [K]o, a dose of ouabain that was stimulatory may become inhibitory. The reversal potential for iK2 then shifts in a positive direction. 5. During either stimulation or inhibition, the speed of change of reversal potential is consistent with a change in [K]e, which may change fairly rapidly. It is not possible to account for the results solely by changes in intracellular concentration, [K]i. 6. Low concentrations of ouabain were found to have no effect on the activation curve, s infinity (Em), controlling iK2. It is concluded that the changes in iK2 are solely attributable to changes in reversal potential. 7. Since net stimulation of the Na+-K+ exchange pump was observed to occur at doses of ouabain that exert a strong positive inotropic action on Purkinje fibres (Blood, 1975), it is not likely that the inotropic action is causally related to net pump inhibition.

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