In epidermal cells of maize (Zea mays L.) coleoptiles, cytosolic pH (pHc), cytosolic free calcium, membrane potential and changes thereof were monitored continuously and simultaneously (pHc/,ψ m, Ca(2+)/ψ m) using double-barrelled ion-sensitive microelectrodes. In the resting cells the cytosolic pH was 7.3-7.5 and the concentration of free calcium was 119±24 nM. One-micromolar indole-3-acetic acid (IAA), added to the external medium at pH 6.0 triggered oscillations inψ m, pHc and free calcium with a period of 20 to 30 min. Acidification of the cytosolic pH increased the cytosolic free calcium. Theψ m oscillations are attributed to changes in activity of the H(+)-extrusion pump at the plasmalemma, triggered off by ΔpH and controlled by pH regulation (pH oscillation). The origin of the pHc and Ca(2+) changes remains unclear, but is possibly caused by auxin-receptor-induced lipid breakdown and subsequent second-messenger formation. It is suggested that the observed cytosolic pH and Ca(2+) changes are intrinsically interrelated, and it is concluded that this onset of regulatory processes through the phytohormone IAA is indicative of calcium and protons mediating early auxin action in maize coleoptiles. It is further concluded that the double-barrelled ion-sensitive microelectrode is an invaluable tool for investigating in-vivo hormone action in plant tissues.