Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31) activity and diurnal changes in malic (Δ-malate) and citric acid (Δ-citrate) contents were determined in salt-treated and continuously well-watered plants of the facultatively halophytic ephemeral Mesembryanthemum crystallinum for a period of approximately 50 days. Twenty-two days after germination, PEPC activity (measured at pH 7 and pH 8) began to increase slightly in salt-stressed and in control plants. This change coincided with a step-wise (≈160%) rise in the ratio of PEPC activity at pH 7 to pH 8 (PEPC-7/8) and with the onset of a low but significant Δ-citrate. After 35 days a second, sudden, three to four-fold augmentation in maximum PEPC activity was observed in plants from all sets. PEPC-7/8 increased by nearly 40% to 0.8 within 4 days and a pronounced Δ-malate appeared for the first time. The appearance of Δ-malate was independent of the duration of the salt treatment and occurred, though to a much lesser extent, also in control plants. The length of the lag-phases between the occurrence of Δ-citrate and Δ-malate, and the two steps of PEPC activity increase, decreased with increase of plant age at the beginning of the salt treatment. This coincidence pointed out the causal interrelationship between these events. The low initial rise in PEPC activity allowed a nocturnal accumulation of citric acid. An accumulation of malic acid required a high PEPC activity and a high PEPC-7/8. Relief from salt stress resulted in a drastically decreased PEPC activity without preventing nighttime malic acid accumulation. PEPC-7/8 also declined slighdy to nearly the level attained by the control plants at the same time. PEPC-7/8 tended to increase continuously in well-watered plants during the later stages of the experiment.