Voltage clamp was used to measure the voltage dependence of cell-to-cell coupling via plasmodesmata between higher-plant cells (root hairs of Arabidopsis thaliana (L.) Heynh.). In addition, ionophoresis was used to introduce a variety of ions [Ca 2+, inositol-trisphosphate, Li +, K +, Mg 2+, ethylene glycol-bis([3aminoethyl ether)-N,N,N' ,N' -tetraacetic acid (EGTA), 1,2-bis(2-aminophenoxy)ethane-N,N,N' ,N' -tetraacetic acid (BAPTA), H +, and OH ] to examine whether they regulate cell-to-cell coupling. Electrical coupling showed high variability in this single cell type at the same developmental stage; the coupling ratio ranged from near 0% to about 90% with a mean value of 32%. It was voltage independent for intracellular voltage gradients (transplasmodesmatal) of -163 to 212 inV. While Ca 2+ closes the plasmodesmatal connections (at concentrations higher than those causing cessation of cytoplasmic streaming), inositol-trisphosphate and lithium are without effect. Apparently, inositol-trisphosphate may not cause increased cytosolic Ca 2+ in root hairs. Alkalinization by OH ionophoresis caused a modest decline in cell-to-cell coupling, as did acidification by H + ionophoresis (to an extent causing the cell to become flacid). Increases in cytosolic K +, Mg 2+, and the calcium chelator BAPTA by ionophoresis had no effect on cell-to-cell coupling. The regulation (and lack thereof) reported here for plant plasmodesmata is quite similar to that of gap junctions.