Abstract The concentrations of transferrin and albumin in the blood serum and microsomal fraction of the liver and the incorporation of [ 14C] leucine into the proteins were measured in rats which were fasted while exposed to ambient atmospheric pressure or to a pressure of one-half atmosphere. The rates of protein synthesis were estimated in a relative manner from the ratio of 14C incorporation into the two proteins and in an absolute manner using the liver free 14C and leucine concentrations to measure the specific activity of the precursor pool. Fasting at ambient pressure was accompanied by a decrease in the serum and microsomal concentrations of transferrin but not of albumin and by a marked decrease in the relative and absolute synthesis rates of transferrin. By contrast, fasting at reduced ambient pressure was associated with an increase in the serum transferrin concentration and in the relative and absolute rates of synthesis of the protein. It is concluded that fasting in the rat produces a much greater decrease in the rate of synthesis of transferrin than of albumin and that exposure to reduced ambient pressure stimulates transferrin synthesis but not albumin synthesis.