Studies on the photorespiration of four typical C3 and C4 plants and six CAM plants showed that C4 and CAM plants showed postilluminaiion CO2 burst (PICB) as did C3 plants. The activity of glycolate oxidase In C4 and CAM plants compared to that in the C plants was much lower and the activity of catalase slightly lower. Glycolate formation also existed in C4 and CAM plants. It was considered that there was a glycolate cycle in C4 and CAM plants. The rate of photorespiration In C, plants may be related to their anatomical features, and as to CAM plants it may be related to the diurnal rhythm of its metabolism. The variation of glycolate cycle in the morning and dusk in CAM plants was studied. It was found that most of the CAM plants showed higher activities both of glycolate oxidase and catalase and a larger quantity of glycolate formation at dusk than in the morning. The turnover of the glycolate cycle was postulated to be faster at nightfall than in the morning in CAM plants.