The half-life of plastoquinone (PQ), ubiquinone-9 (UQ-9) and ubiquinone-10 (UQ-10) in spinach tissue was determined. This was achieved by monitoring the decay of radioactivity incorporated into these lipids from a labeled precursor. The half-life of PQ was 15 h while for UQ-9 and UQ-10 it was longer, i.e. 30 h. The values of half-lives of PQ and UQ suggest a high rate of turnover of these lipids in spinach cells.