The inhibitory neurotransmitter GABA (gamma-aminobutyric acid) has been shown to have a depolarizing action on myelinated axons of both mammalian and amphibian peripheral nerves. In initial in vivo observations intravenous injections of GABA caused an increase in the excitability of the low-threshold, fast conducting fibers of the superficial radial and median nerves of the cat. Similar, graded, reversible effects were confirmed (using changes in the amplitude/integral of the stimulus-evoked A-fiber submaximal compound action potential to assess excitability) in in vitro studies with the isolated, desheathed frog sciatic nerve. GABA caused a mean maximal increase in half-maximal action potential of 29.8% (S.E. +/- 2.7), with an ED50 value of 0.09 mM and Hill coefficient of 0.70. This effect did not appear to desensitize, and could be reversibly antagonized by both bicuculline and picrotoxin. Comparison of agonist sensitivities showed a rank order of potency with muscimol greater than 3-aminopropanesulfonic acid greater than GABA greater than beta-guanidinopropionic acid greater than imidazole-acetic acid greater than guanidoacetic acid greater than delta-aminovaleric acid. With structure activity analysis the maximal activity was found to be related to N+-C separation near the 5 A value. Partial substitution of chloride ions in the superfusate by isethionate reversibly depressed the effect of GABA. These observations support the conclusion that extrasynaptic receptors for GABA are present on the myelinated axons of peripheral nerves.