THE MEASUREMENT OF SYNTHESIS RATES OF ALBUMIN AND FIBRINOGEN IN RABBITS.

1. A formula is proposed for calculating fractional synthesis rates of liver-produced plasma proteins that dispenses with urinary information or information about the size of the urea pool in the body or the fraction of urea that is endogenously catabolized. 2. Synthesis rates obtained for albumin and fibrinogen agreed well with corresponding catabolic rates for the (131)I-labelled proteins except in two of the fibrinogen measurements. 3. Significant reutilization of (14)C occurs in some animals after [(14)C]carbonate injections, giving rise to errors in the calculation of protein synthesis rates. These can best be avoided by using results obtained by injecting [(13)C]urea simultaneously. [(15)N]urea is shown not to be satisfactory for this purpose.