Potassium transport was measured in isolated chicken colonocytes using 85Rb+ as a tracer for K+. Rb+ was determined by atomic absorption spectrometry. The results revealed that net K+ uptake occurred via at least four mechanisms: (i) Na+,K+-ATPase, (ii) K+-ATPase, (iii) Na+-K+-2Cl- cotransport system and (iv) a mechanism(s) which is resistant to both ouabain and bumetanide. The rate of K+(Rb+) efflux is stimulated by the calcium ionophore A23187, inhibited by either quinine, verapamil or Ba2+, and unaffected by either apamin, 3,4-diaminopyridine (3,4-DAP), H2-DIDS or bumetanide. The A23187-induced increase in K+(Rb+) efflux was abolished by apamin. These findings suggest that K+(Rb+) efflux from chicken colonocytes occurs at least in part through Ca2+-activated K+ channels. The present results also show that all these K+ transport systems are involved in cell volume regulation. Thus, external hyposmolarity decreased net K+(Rb+) uptake mediated by Na+,K+-ATPase, K+-ATPase and the Na+-K+-2Cl- cotransporter and increased K+(Rb+) efflux rate. The opposite was observed under hyperosmotic conditions.