1. Some effects of noradrenaline on potassium efflux, electrolyte levels, membrane potential and current distribution in guinea-pig liver slices have been examined.2. The slices (thickness ca. 300 mum) were prepared from the median lobe of the liver and incubated at 38 degrees C in a mammalian Ringer fluid containing 2 mM pyruvate. After an initial recovery period, the ionic composition of the tissue remained stable for several hours.3. The steady-state contents of sodium, potassium and chloride were 296, 266 and 272 m-equiv/kg dry tissue respectively. The inulin space was 29 ml./100 g wet tissue.4. Most if not all of the tissue potassium was exchangeable. The rate constant for (42)K efflux was 0.019 min(-1).5. Noradrenaline (1 muM) markedly increased the efflux of (42)K and within 2 min caused tissue potassium to fall by 8%. At the same time the sodium content rose.6. Traverses of the slices with micro-electrodes showed many negative-going deflexions of 30-40 mV in amplitude. The evidence suggests that these correspond to the membrane potentials of the parenchymal cells.7. Noradrenaline (1 muM) caused a reversible hyperpolarization of about 10 mV. The response became larger on replacing external chloride by isethionate or methylsulphate, but was little affected by a reduction in external potassium.8. After slices had been bathed in potassium and chloride-free solutions for several min, restoration of external potassium caused the membrane potential to increase by up to 10 mV. This hyperpolarization, but not that caused by noradrenaline, was abolished by ouabain.9. Noradrenaline reduced the amplitude and quickened the time course of electrotonic potentials set up by current pulses from another microelectrode, suggesting that the membrane conductance had risen.10. Although certain mechanisms based on electrogenic active transport processes with unusual properties have not been excluded, the present findings are more simply explained by supposing that noradrenaline increases the potassium permeability of the parenchymal cell membrane.