The development of the dorsal root potential (DRP) and the responsiveness of primary afferent fibers to gamma-aminobutyric acid (GABA) were investigated in the isolated spinal cord of rat fetuses. At embryonic day 15.5, stimulation of the lumbar dorsal root was first effective in eliciting the DRP, which was not inhibited by bicuculline. A bicuculline-sensitive component of the DRP appeared at embryonic day 17.5. GABA (10 microM to 1 mM) caused a dose-dependent depolarization of the primary afferent fibers from embryonic day 13.5. The amplitude of the depolarization gradually increased with age until embryonic day 17.5 and was maintained thereafter. If the bicuculline-sensitive DRP solely reflects GABAergic activity, it is suggested that GABAergic activity develops at embryonic day 17.5 and the development of the responsiveness of primary afferent fibers to GABA precedes the functional onset of GABAergic neurons.