The responses of maize grain and dry matter yields to timing and severity of water deficit in a sub-humid environment were studied in the field for two seasons. Seventeen irrigation treatments were applied to maize grown on clay-loam soil, at three critical development stages: vegetative, flowering and grain-filling. The grain and dry matter yields increased with the amount of irrigation water. In both seasons, the highest grain yields were obtained from full irrigation at each stage. Yields were reduced in all the other treatments in which water was limited in all or in part of the development stages. Yield response factor (ky) was separately calculated for the individual growth stages and for the total growing season, and was found to be 0.90, 1.12 (the highest value) and 0.87 (the lowest value) for the total growing season, flowering, and flowering and grain-filling combination stages, respectively. Maximum values of both water use efficiency and irrigation water use efficiency for grain yield under irrigation conditions were obtained as 2.05 kg m(-3) and 1.62 kg m(-3) from treatments of full irrigation at the flowering and grain-filling stages, and from full irrigation at the vegetative and flowering stages, respectively. Full irrigation during the total growing season was found to be the most appropriate choice for maximum grain yield under the local conditions, but these irrigation programs must be reconsidered in areas where water resources are more limited. Our data suggest that water stress should be scheduled on the grain-filling stage in the case of limited water or water scarcity. Withdrawal of irrigation water during the flowering stage was not a good strategy under the conditions of this study.