The effect of environmental conditions, temperature, relative humidity, and light, together with the regulation of PEPC (phosphoenolpyruvate-carboxylase) activity by malate and pH on CAM (crassulacean acid metabolism), was studied in members of the Mesembryanthemaceae in their natural environment, the southern Namib desert. It was found that during a 24 h period the characteristics of PEPC change. Before sunrise the activity is higher when measured at pH 7 than 8. With bright sunlight the activity measured at pH 7 drops to 20% of its pre-sunrise value, the activity only recovers gradually after malate disappearance and stays constant throughout the night. When measured at pH 8, PEPC shows an opposite behavior, i.e., activity increases in bright sunlight and declines as the pH 7 activity increases. A day-night oscillation in the capacity of malate to stimulate or inhibit PEPC was found. During the day malate inhibits about 90% of the PEPC activity at both pH 7 and 8. After sunset there is a sudden decrease in this inhibition and, at pH 8, malate stimulates the activity by 50%. At pH 7 the stimulation was less. Keywords: CAM regulation, Environmental variables, Malate inhibition/stimulation, Mesembryanthemaceae, PEP carboxylase.