The effects of an intravenously-given local anesthetic was studied on impulse conduction in myelinated and nonmyelinated axons. In 13 cats, the right femoral nerve was stimulated proximally, and the compound nerve action potential recorded distally from its saphenous branch. From their respective conduction velocities, [alpha]-, [delta]- and C-fiber responses were identified. Intravenous lidocaine (5.0-17.5 mg/kg) depressed response amplitude more than conduction time in [delta] - and C-fibers; C-fibers were more depressed than were [delta] -fibers. The linear log dose-effect relationship for depression of amplitude and conduction velocity by lidocaine was statistically significant for C-fibers only. Even the largest tolerated intravenous doses of lidocaine depressed the amplitude of the C-fiber potential less than 50%; such doses barely affected conduction in [alpha]-fibers (less than 5%). Recovery of function, slowest in C-fibers, was approximately 2 times faster in [delta] -fibers and took less than 5 min. in [alpha]-fibers. Arterial hypotension produced by large doses of lidocaine evidently was not responsible for changes in nerve parameters; in contrast to the blocking sequence observed with lidocaine, ischemia depressed conduction in [alpha]-fibers and had little immediate effect on C-fibers.