Patch clamp measurements of excised inside-out patches of Arabidopsis thaliana plasma membrane reveal at least two ion channels which conduct either potassium or chloride. The conductance of the potassium channel ranged from 5 to 70 picosiemens depending on KCl concentration. The conductance increased linearly with increasing cytoplasmic-side [KCl]; the extent of this dependence declined as extracytoplasmic-side [KCl] was increased. This indicates that substrate regulation of the potassium channel is a consequence of the molecular architecture of the channel: in particular, multi-ion binding sites within the channel pore. The chloride channel conductance (ranging from 5-40 picosiemens) was independent of cytoplasmic-side [KCl] until a threshold concentration of about 300 millimolar was reached. Such behavior is expected only if the channel is allosterically regulated by cytoplasmic-side K(+) and/or Cl(-). The median open times of either channel (about 200 milliseconds for the potassium channel and 20 milliseconds for the chloride channel) were unaffected by substrate concentrations.