Liver regeneration in the rat following partial hepatectomy (PH) is a frequently used model to study regulatory mechanisms in relation to cell growth and differentiation. In the present study, we analysed quantitative changes in the peripheral circulation of a number of important serum proteins following PH and laparotomy. Alpha-fetoprotein synthesis was induced in the PH rats and remained at very low levels in non-operated controls. Pregnancy-associated murine protein-1 levels and fibronectin levels were lower in the circulation of all operated animals compared with the intact controls. The serum levels of alpha-2 macroglobulin were elevated in all operated animals compared with the non-operated controls. Circulating complement factors C3c and C4 were present at significantly lower levels in PH animals than in rats following laparotomy. Small growth stimulating molecules (<10 kDa) synthesized by different organs during the regenerative process have been described in the literature, but dialysates of the spleen and liver from non-operated rats and PH rats showed no significant impact on serum levels of the proteins in the present study.