Summer drought is a major factor limiting the regrowth of perennial ryegrass seed crops. This phase of crop development has a strong influence on seed yield because most of the tillers that contribute towards next season's seed crop are produced or regrown during this period. In recent years many seed fields have exhibited premature decline probably because of extensive drought after harvesting. Therefore, this study was undertaken to (i) assess how post-harvest leaf and tiller development is impacted by the timing and severity of water stress and (ii) identify potential relationships of water stress to flowering and seed yield. Rain-out shelters were used to exclude rainfall from two cultivars which received either no irrigation or 2.5 cm of simulated rainfall in mid-August or mid-September or both. These were compared to an ambient treatment. No rainfall decreased total tiller production by approximately 30% in 1995 and 50% in 1996. There were also moderate reductions in tiller dry weight, tiller height, and slight decreases in number of leaves and the basal diameters. The trend showed that the cultivar Affinity responded quicker to an early irrigation whereas the cultivar Buccaneer had a longer period of summer dormancy. Total soluble sugars concentrations increased as tiller number decreased suggesting the potential for rapid compensatory growth upon alleviation of drought. Limited irrigation during the post-harvest period of regrowth did not generally affect fertile tiller number nor seed yields. Fewer vegetative tillers as the stands aged, together with other changes in plant parameters, may mark the beginning of the dieback problem. In greenhouse studies, four cultivars were rapidly stressed using vermiculite as a growth medium. Though little differences were observed among cultivars when physiological responses were evaluated, stomata' diffusive resistance and leaf temperatures increased, whereas plant water potential and leaf transpiration decreased as stress was prolonged. Plant survival following water stress was largely dependent on cultivar and gravimetric water content of vermiculite. This technique did not reasonably simulate natural drought conditions in terms of plant physiological performances nor soil characteristics, but it was useful to differentiate the ability of different genotypes to survive a drought-induced dieback. / Graduation date: 1998