Glycolate oxidase is one of the key enzymes in the pathway of photorespiration. In this study we investigated the effects of light on the expression of the spinach glycolate oxidase gene. Continuous exposure to white light resulted in a gradual increase in the steady-state level of glycolate oxidase mRNA within a time period of 2~24 h in both etiolated and dark-adapted green seedlings. A short white light pulse also increased the level of glycolate oxidase mRNA in etiolated seedlings. The mRNA level reached a maximum at 6~8 h after the pulse and decreased by 24 h after the pulse. The induction patterns of the glycolate oxidase gene by white light appeared similar to those of the rbcS gene, indicating that a common or coordinating regulatory system may be involved in the expression of the glycolate oxidase and rbcS genes. A red light pulse induced an increase in the amount of glycolate oxidase mRNA and this effect was reversed by a subsequent far-red light pulse, suggesting that the expression of the glycolate oxidase gene is regulated by phytochrome.