A number of environmental stimuli can bring about changes in the aperture of the stomatal pore via alterations in guard cell turgor. The planthormone abscisic acid (ABA) and high CO2 both inhibit stomatal opening and promote stomatal closure. We have been testing the hypothesis that the Ca*+ ion is a common component in the signal transduction pathways by whkh ABA and CC^ bring about stomatal closure. This has been tested directly by measuring alterations in guard cell cytosolic free calcium ([Ca*+];)in response to 700 ppm COj. [Ca*+]i was measured using ratio photometry and digital ratio imaging following microinjection of the fluorescent calcium indicator, fura-2, directly in to the cytosol of the guard cell. Pharmacological data also indicate a role for calcium in the stomatal response to CO2.