Net K+ fluxes in isolated hamster brown fat cells were studied by the use of the K+ analogue 86Rb+. In isolated cells, cold-stored overnight to diminish K+ gradients, an equilibrium 86Rb+ (K+) clearance value of 27 microliter/million cells was obtained after 30 min incubation at 37 degrees C. This corresponds to a 10-fold K+ gradient over the plasma membrane, and a K+ potential of about -60 mV. The attainment of this equilibrium was dependent upon the presence of Na+ in the extracellular medium, and the uptake was fully inhibited by the (Na+ + K+)-ATPase inhibitor ouabain. Ouabain had, however, no significant acute effect on the maximal rate of thermogenesis achieved after norepinephrine stimulation of the cells, but if the restoration of ionic equilibrium was inhibited by ouabain in prolonged incubations, a decreased thermogenesis was observed. This was probably due to the low cytosolic K+ content then encountered, and the resulting inhibition of lipolysis. The addition of norepinephrine to cells in which 86Rb+ (K+) equilibrium had been attained resulted in a rapid efflux of 86Rb+ and the establishment of a new equilibrium value, at about 65% of the unstimulated value. This corresponds to a decrease in K+ potential of about 15 mV. The effect of norepinephrine was stereospecific and reversible, and had an EC50 value of about 10 nM. As catecholamine effects were much more sensitive to phentolamine than to propranolol, the adrenergically-induced efflux was classified as predominantly alpha-adrenergic. It is suggested that the norepinephrine-induced K+ efflux is due to a (probably Ca2+-mediated) opening of K+ channels in the cell membrane, and that this effect occurs secondarily to the alpha-adrenergically induced membrane depolarization (and increase in cytosolic Ca2+). The increased PK over the cell membrane would counteract further depolarization, and the K+ gradient would then approach the Nernst equilibrium under the new steady-state conditions.