The rate of protein degradation in Zea mays leaves has been estimated by using tritiated water and [(3)H]acetic anhydride as the labeling agents. Both methods circumvent many of the problems usually associated with measuring protein degradation in plants. The half-life of ribulose-1,5-bisphosphate carboxylase protein in second leaves of 13-day-old seedlings under continuous light was found to be 7.8 +/- 0.9 days by the tritiated water technique and 6.5 +/- 0.8 days by the [(3)H]acetic anhydride method. The half-lives determined under a 14-hour-light, 10-hour-dark photoperiod are 6.2 +/- 0.8 days with tritiated water and 5.4 +/- 0.4 days with [(3)H]acetic anhydride. Whereas the values obtained by the two methods do not differ significantly, the use of either method for the determination of protein half-life can be recommended.