A field study of the variation in leaf water potential (.psi.L), leaf conductance (gL) and other aspects of crop water relations in a set of 10 spring wheats is described. The 2 main soil moisture regimes used were irrigation and drought; the latter being imposed by placing rainwater gutters between the crop rows to intercept a proportion of any rainfall. The development of plant stresses, the changes in leaf conductances during the growing season and the relationship between these variables and soil moisture depletion are described. Mean daytime leaf water potentials declined during much of the season even in well irrigated plots. There was also evidence that stomata tended to be most open during the period just prior to anthesis. Data on variation of .psi.L and gL within the crop are also presented and indicate that most evaporation occurs from the flag leaf in the spring wheat canopy. Although genotypic variation was found for the mean values of .psi.L and gL, interactions between variety and water regime were generally non-significant. Although yields were not well correlated with most aspects of crop water relations measured, partly because of the dominance of other factors such as maturity and disease susceptibility, a strong negative correlation was observed between yield and gL for the period 7-14 days before anthesis. Stomatal frequencies were not related to leaf conductances and there was no evidence that the stomatal behavior adapted to previous drought stress. The growth responses observed could be regarded as adaptations for reducing evaporation. The results are discussed in relation to the breeding of drought tolerant varieties of wheat.