Canopy conductance of irrigated and unirrigated pearl millet plants was measured with a diffusive resistance porometer in a field experiment in Central India. When plants were growing in a drying soil, canopy conductance was related linearly to the amount of light intercepted by the canopy, and was unaffected by large changes in atmospheric saturation deficit When water was given to other plants growing in the same field, canopy conductance became strongly influenced by changes in saturation deficit.