1. The resting membrane potentials of proximal and distal segments of the arterioles which arise from the rat middle cerebral artery were determined. Proximal segments had stable membrane potentials with a mean value of -69 mV. The membrane potentials of distal segments were less negative and often unstable. 2. When the extracellular concentration of potassium ions [( K+]o) was increased proximal segments of arteriole were depolarized whereas distal ones were hyperpolarized. When [K+]o was decreased both proximal and distal segments were depolarized, the changes being more marked in proximal arterioles. 3. The membranes of proximal segments of arteriole displayed inward rectification at potentials near rest; inward rectification in distal segments of arteriole, when detected, was less pronounced. 4. The activation curve for inward rectification in proximal segments of arteriole was changed by changing the extracellular concentration of K+. A reduction in [K+]o caused the activation curve to move to such negative potentials that the inward rectifier no longer contributed to the resting conductance. 5. Increasing [K+]o changed the activation curve for inward rectification in distal segments of arteriole so that more K+ current flowed at potentials near resting. At the same time the membrane potential hyperpolarized. 6. The results are discussed in relation to autoregulatory changes which occur following changes in the K+ concentration of cerebrospinal fluid.
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