The stimulus responsible for inducing hepatocytes to enter the cell cycle following partial hepatectomy (PHx) remains to be identified. One suggested candidate is the change in hepatocyte membrane potential (PD) that occurs immediately following PHx. To test this possibility, we monitored changes in hepatocyte PD and immediate-early proto-oncogene expression in rats pretreated with saline or gamma aminobutyric acid (GABA), an amino-acid neurotransmitter that hyperpolarizes isolated hepatocytes. Intraperitoneal injections of saline or GABA (500 μg/g body weight) were administered to adult, male Sprague-Dawley rats 1 hour prior to 70% PHx. Rats were sacrificed and the livers excised at various times until 180 minutes post. PHx for messenger RNA (mRNA) and protein analyses. In additional groups of saline- and GABA-treated rats, PD changes were recorded continuously from 260 to 180 minutes post-PHx. Serum GABA concentrations were determined by ion-exchange chromatography with orthopthaldehyde fluorescence detection. Hepatocyte PD's were recorded in situ by intracellular microelectrodes with an Axoprobe-1A amplifier. Steady-state levels of c-fos, c-jun, jun-B, and c-myc transcripts and proteins were documented by Northern blots of poly(A)-enriched RNA derived from resected livers and Western blots of total nuclear protein, respectively. Serum GABA concentrations remained unchanged in saline-treated controls but increased 10- to 20-fold above baseline values in GABA-treated rats. In saline-treated controls, hepatocyte depolarization occurred immediately and was maintained throughout the 180 minutes post-PHx period (PD pre-PHx, -36.8 ± 5.1; 15 minutes post-PHx, 27.5 ± 5.7; and 180 minutes post-PHx, 28.3 ± 4.4 mV, mean ± SD); whereas in GABA.treated rats, hepatocyte PD remained unchanged (-37.0 ± 1.1; -36.4 ± 3.1 and -39.2 ± 2.7 mV, respectively). Despite abrogation of hepatocyte PD changes, proto- oncogene mRNA and protein levels in saline- and GABA-treated rats were either similar or, in the case of c-fos and c-jun, increased five- to sevenfold in GABA-treated rats. The results of this study indicate the following: 1) hepatocytes depolarize immediately post-PHx and remain depolarized throughout the priming phase of the cell cycle; 2) elevated serum GABA concentrations prevent PHx-induced hepatocyte depolarization; and 3) depolarization is not the stimulus responsible for priming hepatocytes into replicative competence.