1. Swards of perennial ryegrass (S 24) were grown under conditions of high fertility and were cut at intervals of (a) 3 weeks, (b) 6 weeks, (c) 3 weeks changing to 6 weeks at later points in the growing season.2. The rate of herbage production (kg D.m./ha/day) was higher throughout the season (April to November) under the longer cutting interval. Plots which were changed from 3- to 6-weekly cutting intervals subsequently showed the same rate of production as those cut every 6 weeks throughout.Inflorescences emerged from a higher proportion of tillers after long growth periods; this could explain the higher rates of production up to July.