The response of three grain crops to drought was determined in a series of experiments in a mobile rainshelter at Lincoln, Canterbury, New Zealand. For winter wheat and spring barley, the critical potential soil moisture deficit (Dc) beyond which yield was affected was independent of drought timing, but was much smaller for barley than wheat. Both crops showed similar yield responses to drought above their Dc. In contrast, Dcfor maize increased as the season progressed, and yield was much less sensitive to drought once Dcwas exceeded. Although yield response was mostly associated with grain number in wheat and barley, mean kernel mass was also reduced as drought stress increased. Grain yield in maize was correlated with mean kernel mass, and uncorrelated with grain number.