Irrigation experiments with grain sorghum and maize conducted at Davis on a clay loam of open structure and great depth, showed that the extent of yield reduction following evapotranspiration deficits depended on the growth period during which these occurred. With sorghum, deficits were least tolerated in the vegetative and pollination periods. The same held for maize but this crop could be conditioned to withstand deficits at mid-season to some extent by restriction at an earlier stage. In practice, the ground water potential at the beginning of the season was decisive. It was concluded that economic optimization of water use, under sub-optimum conditions, must take account of the variable effects of water deficit at different stages of growth, particularly for maize