Ageing in tail tendon collagen was studied in 7 groups of male Wistar rats eating respectively 3.5, 7, 10.5, 14, 17, 21, and 24 g food/day. Body and tail tendon weights increased in proportion to food intake. The maximum body weight was approximately 20 times the daily food intake and the tail tendon weight 3 % of the food intake. The ageing of collagen, as measured by the breaking time in 7 m urea at 40°C, was retarded by food restriction. The greatest retardation occurred in rats eating 10.5–14g food/day (50–70% of the mean ad libitum intake). It was shown that the ageing of collagen is relatively independent of food intake below the threshold of 70% ad libitum intake. Above this level the excess of food accelerated the ageing of collagen in tail tendon.