Phosphoenolpyruvate carboxylase (PEPCase; EC 4.1.1.31) activity was found to be modulated by light and darkness when measured in the presence of K(+), which had been added to induce swelling of guard-cell protoplasts (GCPs) from Vicia faba L., whereas no modulation was detected in the absence of K(+) (PEPcase activity remained constant at 1.5±0.15 pmol PEP metabolized · GCP(-1) ·h(-1); subsequently, pmol GCP(-1) ·h(-1) will be used). The activity of PEPCase increased by 100% (from 1.5 to 3 pmol·protoplast(-1)·h(-1)) in darkness and by 200% (from 1.7 to 5 pmol·protoplast(-1)· h(-1)) in light and oscillations in activity of these magnitudes were repeated at intervals of 2 min (dark) and 2.5 min (light) for a period of 10 min during K(+)-induced increase in the volume of GCPs. The oscillations were reflected in changes in malate-pool sizes determined in plastids, mitochondria and the supernatant fraction (consisting of the cytosol and the vacuole). Malate probably functioned as a mitochondrial substrate, thus supplying ATP for K(+) uptake and the swelling of the protoplasts. On the basis of the present paper and previous results (H. Schnabl and B. Michalke 1988, Life Sci. Adv. Plant Physiol. 7, 203-207) involving adenine nucleotidepool sizes in fractionated GCPs, a model is proposed to explain the cause-effect relationship between K(+), PEPCase, the cytosolic and mitochondrial malate levels and ATP levels during the K(+)-induced increase of GCP volume.