To assess the extent of water flow through channels in the membranes of intact higher plant cells, the effects of HgCl2 on hydraulic conductivity (LP) of wheat (Triticum aestivum L.) root cells were investigated using a pressure probe. The LP of root cells was reduced by 75% in the presence of 100 μm HgCl2. The K+-channel blocker tetraethylammonium had no effect on the LP at concentrations that normally block K+ channels. HgCl2 rapidly depolarized the membrane potential (Vm) of the root cells. The dose-response relationship of inhibition of LP and depolarization of Vm were not significantly different, with half-maximal inhibition occurring at 4.6 and 7.8 μm, respectively. The inhibition of LP and the depolarization of Vm caused by HgCl2 were partially reversed by β-mercaptoethanol. The inhibition of LP by HgCl2 was similar in magnitude to that caused by hypoxia, and the addition of HgCl2 to hypoxia-treated cells did not result in further inhibition. We compared the LP of intact cells with that predicted from a model of cortical cells incorporating water flow across both the plasma membrane and the tonoplast using measured values of water permeability from isolated membranes, and found that HgCl2 has other effects in addition to the direct inhibition of water channels.