Soybean physiological and yield responses to water stress were studied in a movable weather (rainout) shelter facility at Ames, Iowa, during the summer of 1978. Soil-moisture deficits were induced in six combinations of three stress periods during reproductive growth to investigate the possibility of physiological and yield enhancement in response to preconditioning for later water stress. Preconditioning did result in enhanced stomatal conductance, but the enhancement could not be attributed to either avoidance or toleration of low leaf water potentials. Cultivar variations in physiological responses to preconditioning were not detected;Cultivar yield differences in response to the various stress treatments resulted primarily from stress timing although evidence for less yield sensitivity to water stress was observed in some cultivars. Total plant yield reductions were approximately equally distributed on the upper 75 cm of stems and on the branches. Preconditioned plants had less yield reduction during later stress periods than did unconditioned plants which were stressed for the first time. Cultivar yield differences in response to preconditioning seemed to result from stress timing effects on unconditioned plant yields rather than on cultivar responses to preconditioning;Although the stress-induced yield reductions were closely related to above-ground dry-weight losses for each of the stress treatments, stress early in reproductive growth resulted in greater dry-matter partitioning to the seeds. The earlier the first stress occurred in reproductive growth, the greater was partitioning to the seeds; consequently, mild stress during the early flowering period of indeterminate soybeans could conceivably increase yields through partitioning effects. Preconditioned plants received their pre-stress treatment early in reproductive growth, so their partitioning to the seeds was greater than unconditioned plants;Investigations on the need for weighting of a soil-moisture stress index for growth stages revealed a curvilinear yield response to the accumulated stress. This curvilinear response was related to a very high low-stress-treatment yield for the highest-yielding cultivar. Stress during early beginning-seed filling caused less yield reductions than predicted from the stress index and may need to be weighted for better yield predictions.