In cells of Zea mays (root hairs, coleoptiles) and Ricciafluitans (rhizoids, thalli) intracellular Ca 2 + and pH have been measured with doublebarrelled microelectrodes. Free Ca 2+ activities of 109-187 nM (Riccia rhizoids), 94 160 nM (Riccia thalli), 145-231 nM (Zea root hairs), 84-143 nM (Zea coleoptiles) were found, and therefore identified as cytoplasmic. In a few cases (Riccia rhizoids), free Ca 2 + was in the lower millimolar range (2.3 _+ 0.8 mM). A change in external Ca 2+ from 0.1 to 10 mM caused an initial and short transient increase in cytoplasmic free Ca 2 + which finally levelled off at about 0.2 pCa unit below the control, whereas in the presence of cyanide the Ca 2 + activity returned to the control level. It is suggested that this behaviour is indicative of active cellular Ca 2 + regulation, and since it is energy-dependent, may involve a Ca 2 +-ATPase. Acidification of the cytoplasmic pH and alkalinization of the vacuolar pH lead to a simultaneous increase in cytoplasmic free Ca 2+, while alkalinization of pHc decreased the Ca 2+ activity. Since this is true for such remote organisms as Riccia and Zea, it may be concluded that regulation of cytoplasmic pH and free Ca 2 + are interrelated. It is further concluded that double-barrelled microelectrodes are useful tools for investigations of intracellular ion activities in plant cells. Key words: Calcium (free, and pH) -Cytoplasm (free Ca 2+, pH) -Microelectrode, double-barrelled, Ca 2 + -pH(interaction with Ca 2 +) --Riceia -Zea (free Ca 2 + and pH)