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Either 86Rb or 42K appears to be a useful marker for monitoring the movement of cellular K in dispersed guinea-pig hepatocytes. Alpha-adrenergic stimulation of perifused hepatocytes causes a biphasic increase in 86Rb or 42K efflux from hepatocytes previously equilibrated with radio-isotope. The first phase is a large transient (about 5 min) increase which is followed by a slowly falling phase of release. Alpha-adrenergic stimulation of hepatocytes perifused with medium containing no added Ca plus 0.1 mM-EGTA evokes only the transient increase in 86Rb efflux. The addition of Ca to the medium in the continued presence of agonist restores the second phase of the response. Both phases of the response appear to be mediated by alpha 1-receptors. The magnitude of the second phase is dependent upon the concentration of Ca added to the perifusion medium. Other agonists that are believed to act by mobilizing Ca give similar results in this system. Angiotensin II, ATP and A23187 stimulate a transient increase in 86Rb efflux without extracellular Ca present, with the second phase of the response appearing upon the addition of Ca to the medium. These results suggest that the initial transient phase of 86Rb efflux, which is independent of extracellular Ca, is stimulated by Ca released from an intracellular pool. The second phase, which occurs only in the presence of extracellular Ca, is probably a result of Ca influx into the cell.