this work characterizes parameters of ubiquitin metabolism in chlamydomonas reinhardtii dangeard growing under constant conditions and after an exposure to cold shock. ratio of free and conjugated ubiquitin to total protein, and rate constant of ubiquitin synthesis and conjugation increased about two-fold during first 4 hours after cold treatment, whereas rate constant of ubiquitin degradation reached its maximum 9 hours after treatment. half-life of ubiquitin calculated from the constant of degradation decreased from 6 hours to 3.5 hours during first four hours after the cold treatment. rate constant of ubiquitin deconjugation did not change after cold treatment. ratio of free to conjugated ubiquitin decreased temporarily to approx. 8 immediately after cold treatment and raised back to its original value at 2 h after cold treatment. these observations raise questions regarding the regulatory mechanisms of ubiquitin synthesis and hydrolysis.