A method for the calculation of rate constants of degradation of a specific protein during a period of change in protein amount is described. The calculation uses measurements of rates of synthesis and amount of the protein, together with one estimate of the protein half-life. The method is demonstrated by using data from measurements made on the accumulation of fatty acid synthetase in hormonally treated explants of mammary gland from mid-pregnant rabbits, and data from other authors. These calculations demonstrate a regulated programme of co-ordinated changes in rates of synthesis and degradation during a period of change in protein mass. The role of such changes in protein degradation during protein accumulation is discussed.