The concentration of plasma haemopexin was significantly increased 24 and 48 h following both partial hepatectomy and laparotomy in the rat. Increased haemopexin synthesis was demonstrated by studies on the in vivo incoporation of isotope labelled amino acids and in vitro perfusion of the isolated liver. In vivo studies also indicated increased synthesis of plasma fibrinogen. No change was found in the incorporation of amino acids into plasma albumin apart from a transient rise 4 h after laparotomy but albumin production by the perfused rat liver 48 h after partial hepatectomy appeared enhanced. The observed changes in rat haemopexin and fibrinogen synthesis may represent a nonspecific acute phase response to surgical trauma.