Induction of Bcl-2 and Bcl-x has been demonstrated in mitogen-stimulated lymphocytes in vitro, suggesting that these two apoptosis modulators may also play a role during proliferation. To explore this possibility in a physiological setting, mRNA expression of various Bcl-2 family members was examined during liver regeneration induced by partial hepatectomy, a well characterized in vivo model of cell cycle progression. After a 60% partial hepatectomy in C3H/HeN mice, the steady-state levels of Bcl-x mRNA exhibited a cyclical pattern, with peaks at 4 hours (early G1) and 48 to 72 hours (G1 phase of the second hepatocyte cell cycle). A1 and Bcl-2 mRNA were not detected, and the levels of two Mcl-1 mRNA species remained low without significant changes. The three pro-apoptotic members of the family, Bak, Bad, and Bax, all showed an early decline in mRNA levels when Bcl-x transcripts increased, followed by later peaks at 12, 24, and 48 to 72 hours, respectively. Experiments were subsequently conducted in C3H/HeJ mice, an endotoxin-resistant strain with slower liver regeneration marked by a protracted G1 phase. Even though immediate-early gene responses measured by c-myc induction remained intact, the timing of Bcl-x mRNA expression was delayed in C3H/HeJ mice. When C3H/HeN mice were pretreated with cycloheximide before hepatectomy, the early peak of Bcl-x mRNA at 4 hours was essentially abrogated whereas the immediate-early gene c-myc was hyperinduced, thus implicating Bcl-x as a delayed early response gene during liver regeneration. Bcl-x was localized in hepatocytes and by both immunohistochemistry and Western blot analysis, Bcl-xL protein reached highest levels at 12 hours (mid-G1), consistent with the expression of a delayed early gene. In summary, the expression profiles of Bcl-2 family members during liver regeneration suggest a cell-cycle-dependent regulation as well as a physiological role for these apoptosis-modulating genes during growth and proliferation.