Mature leaves of well-watered Peperomia camptotricha show Crassulacean acid metabolism (CAM). Young leaves show CAM-cycling in which CO(2) uptake occurs during the day concomitant with a marked diurnal fluctuation of organic acids as in CAM. Evidence is presented suggesting that respiration is the source of CO(2) for nocturnal acid synthesis in leaves exhibiting CAM-cycling. Respiratory quotients for these leaves were consistently much less than unity despite the fact that the leaves metabolize starch. The conservation of CO(2) by refixation into acids at night represents about 17% of the total photosynthetically fixed CO(2) and about 50% of the total respiratory CO(2).