In order to study the relationships betwe e n the shoot growth and the root growth of Italian ryegrass, dry weight T/R ratio and relative growth coefficient (logarithmic growth rate of the root over that of the shoot) of defoliated and not defoliated plants were investigated under several environmental conditions. The following results were obtained; 1) After several defoliation treatments, dry weight of shoot increased immediately but dry weight of root decreased at first and then increased. 2) T/R ratio decreased by defoliation treatments because of the loss of large part of shoot, but then recovered, and reached, the level of that of not defoliation treatment within 20 days after defoliation. 3) Relative growth coeffitients of defoliation treatments became constant in 5 to 10 days after defoliation, but these values were smaller than those of not defoliation treatments. In comparison with defoliation and not defoliation treatments, effect of different environmental conditions on the pattern of dry weight, T/R ratio and relative growth coefficients was very small.