Abstract: An effort has been made to investigate the characteristics of herbage production and water use of perennial ryegrass population under different types and levels of drought stress in field conditions. The experimental material was cv. AberElan, and was grown in self-designed soil columns in May 1995. The columns were packed with soils taken from the field. The soil water content was monitored by regular weighing of the column and was maintained within a predetermined range by controlling watering frequency and amount of water applied each time. Three types of drought stress were imposed to the columns, i, e. , cyclic, sustained and progressive, each of which was further divided into mild, moderate and severe levels, and a well-watered column was used as control. Under cyclic and sustained types of drought stress, the effect of moderate stress on dry matter yield was similar to that of mild stress and was not significantly different from the control, while severe stress reduced dry matter yield; under progressive type of drought stress, however, mild stress did not affect dry matter yield, but moderate and severe stresses caused a reduction in dry matter yield. For each type of drought stress ,dry matter content of herbage was negatively associated with the dry matter yield. There was a linear relationship between dry matter yield and water loss from soil columns. The moderate stress of cyclic type tended to increase water use efficiency(WUE)of the population whereas under sustained and progressive types of stress the response of the population WUE to drought stress varied with time in the growing season.