Several studies have emphasized the involvement of transcription factors, cytokines, and proto–oncogenes in initiating the regenerative process after partial hepatectomy. To assess whether these events do specifically occur in a cellular system undergoing regeneration, we studied the induction of nuclear factor κB (NFκB), interleukin–6 (IL–6), signal transducers and activators of transcription 3 (Stat3), c–fos, c–myc, c–jun, after portal branch ligation (PBL), which produces atrophy of the deprived lobes (70% of the liver parenchyma), whereas the perfused lobes undergo compensatory regeneration. Nuclear extracts and total RNA were prepared from control livers as well as from atrophying and regenerating lobes at 0.5, 1, 2, 5, and 8 after PBL. NFκB and Stat3 induction were studied by electrophoretic mobility shift assays and Western blotting. IL–6 and proto–oncogenes expressions were assessed by reverse transcription polymerase chain reaction and Northern blotting, respectively. Assays were also performed after a sham operation. NFκB and Stat3 protein expression and DNA binding were rapidly and similarly induced in nuclear extracts from the atrophying and regenerating lobes. IL–6 was elevated in both lobes from 1 to 8 hours after PBL as well as c–fos, c–myc, and c–jun during the first 2 hours. IL–6 and Stat3 but not NFκB were also elevated after a sham operation. These findings suggest that the cellular and molecular changes occurring early in a regenerating liver are nonspecific, possibly stress–induced, cellular responses. They do not indicate the future evolution towards atrophy or regeneration.