The effects of sodium pentobarbital on automaticity and conduction were studied in dogs anaesthetized with three different agents. Pentobarbital was administered selectively through the nutrient arteries of the sinus node and AV junction. Interplay with directly administered and neurally released autonomic neurotransmitters was assessed. The electrophysiological effects were exclusively negative, impairing both impulse formation and conduction, independent of the anaesthetic chosen.