Guard cells are a valuable model system for the study of photoreception, ion transport, and osmoregulation in plant cells. Changes in stomatal apertures occur when sensing mechanisms within the guard cells transduce environmental stimull into the ion fluxes and biosynthesis of organic solutes that regulate turgor. The electrical events mediating sensory transduction in guard cells can be characterized with a variety of electrophysiological recording techniques. Recent experiments applying the patch clamp method to guard cell protoplasts have demonstrated activation of electrogenic pumps by blue and red light as well as the presence of potassium channels in guard cell plasmalemma. Light activation of electrogenic proton pumping and the ensuing gating of voltage-dependent ion channels appear to be components of sensory transduction of the stomatal response to light. Mechanisms underlying stomatal control by environmental signals can be understood by studying electrical events associated with ion transport.