The effects of combinations of nitrogen rates and irrigation regimes on yield and seasonal water- and nitrogen-use efficiencies of wheat on deep alluvial loamy sand and san y loam soils with low organic carbon content were studied. Nitrogen application not only increased evapotranspiration and its transpiration component but also increased water-use efficiency of biomass production (based on transpiration). There was a significant positive interaction between applied nitrogen and water supply for their effects on wheat yields. For the pooled data of two soils, a polynomial expressing grain yield as a quadratic function of nitrogen and water supply and their interaction explained 92% of the variation in yield. Isoquants based on this equation showed that within certain limits, nitrogen and water supply substituted for each other in increasing wheat yields.