A red background illumination synergistically increased the sensitivity of the stomatal conductance response to low intensity blue light in wheat seedlings (Triticum aestivum L. cv. Starke II, Weibull). It also saturated the photosynthesis dependent mechanisms so that they did not respond to the low quantum fluence rates needed to activate the blue light mechanism. Thus dual beam experiments provide a suitable experimental approach to study the blue light mechanism for stomatal regulation in the intact leaf. Time course studies indicated that in short time experiments the enhancement effect of the red background light was primarily a result of the increase in conductance level rather than a direct effect of the red light. This was confirmed with experiments where the stomatal blue light response was enhanced due to partial stomatal opening in CO2 free air as well as due to circadian rhythm. During long term experiments the response to blue light gradually decreased. It could then, however, be restored by a simultaneous red background light.