1. 1. Changes in plasma protein synthesis, occuring during the process of liver regeneration, have been investigated using the isolated rat-liver perfusion technique. For this purpose livers from normal, laparotomized and partially hepatectomized rats have been used. 2. 2. Measurements of the relative incorporation of [ 14C]leucine into plasma albumin, fibrinogen and transferrin, show that both partial hepatectomy and, to a lesser extent laparotomy, increase the fractional synthesis rates of both fibrinogen and transferrin relative to that of albumin. 3. 3. The data further suggest that these altered relative fractional synthesis rates reflect diminished synthesis of albumin and increaed synthesis of fibrinogen and transferrin during regeneration, and increased synthesis of fibrinogen and transferrin as a result of laparotomy. 4. 4. The metabolic alterations due to the 2 surgical procedures are further differentiated by an immediate increae in the labelling of liver proteins following partial hepatectomy but not laparotomy.