The mRNA expression of two activin growth factor subunits (betaA- and betaC-activin), activin receptor subunits (ActRIIA, ActRIIB) and the activin-binding protein follistatin, and peptide expression of betaA-activin and betaC-activin subunits, were examined in regenerating rat liver after partial hepatectomy (PHx). Liver samples were collected from adult, male Sprague-Dawley rats, 12-240 h (n=3-5 rats per time point) after PHx or from sham-operated controls at the same time points. Hepatocyte mitosis and apoptosis were assessed histologically and by in situ cell death detection. RT and PCR were used to assess relative gene expression. betaA- and betaC-activin peptide immunoreactivity was assessed in liver and serum samples by western blotting, whereas cellular expression was investigated by immunohistochemistry, using specific monoclonal antibodies. betaA- and betaC-activin mRNA dropped to < 50% of sham control values 12 h after PHx and remained at this level until 168 h post-PHx, when betaA-activin expression increased to three times sham control values and betaC-activin mRNA returned to pre-PHx levels. A peak in follistatin expression was observed 24-48 h post-PHx, coincident with an increase in hepatocyte mitosis. No changes were observed in ActRIIA mRNA, whereas ActRIIB expression paralleled that of betaA-activin mRNA. betaC-activin immunoreactive homo- and heterodimers were observed in regenerating liver and serum. Mitotic hepatocytes frequently contained betaC-activin immunoreactivity, whereas apoptotic hepatocytes were often immunoreactive for betaA-activin. We conclude that betaA- and betaC-activin subunit proteins are autocrine growth regulators in regenerating liver and when expressed independently lead to hepatocyte apoptosis or mitosis in a subset of hepatocytes.