Three feeding experiments and a digestibility study are reported in which the effects of supplementing low nitrogen roughages with roughages of high nitrogen content were studied. In each feeding experiment the effect of varying the frequency of supplementation was examined. During the first experiment intake of lucerne hay was greater (P < 0.05) than the intake of oaten hay and higher wool growth rates and liveweight changes were recorded. When the oaten and lucerne hays were alternated at intervals of one, two, three or four days, the average intake and rate of liveweight change was intermediate between the two previous levels, but the rate of wool production was similar to when lucerne hay was offered alone. There were no differences in total intake, clean wool production or liveweight change due to the frequency with which the two rations were alternated. The second experiment involved the feeding of a fixed quantity of lucerne hay with a. five times as much oaten hay or b. ad libitum oaten hay. The lucerne hay was given daily and every second, fourth and eighth days, and there was an unsupplemented control group. During the restricted feeding period frequent supplementation resulted in higher liveweight gain but there was no effect on wool growth. When oaten hay was provided ad libitum the poor response to supplementation and the large variation in intake within groups resulted in no significant differences in wool production or liveweight change between groups. The results of the third experiment indicate that the digestibility or degree of lignification of the basal ration affects the likelihood of a response to supplementation. It was shown that frequent supplementation of pasture hay resulted in higher intake, liveweight and wool production. On the other hand, substitute feeding with little change in animal production occurred when lucerne hay was offered to similar sheep consuming a basal ration of oaten hay which had a higher level of fibre and a lower in vitro digestibility than the pasture hay.