Two experiments, involving a total of 112 sheep, are described, in which ewes at different reproductive states were given complete diets with a varying grain: straw ratio. In Expt 1 ewes at three reproductive states (dry, pregnant and lactating) were given diets with four levels of coarsely ground oat straw (10, 25, 40 and 55% straw). In Expt 2 ewes in five reproductive states (early pregnancy, mid pregnancy, late pregnancy, lactation and dry) were given a basal diet similar to that given in Expt 1 but with five levels of straw (15, 25, 36, 45 and 55% straw).The results showed that the intake of ewes varied with reproductive state, the highest intakes occurring during lactation. No significant diet x reproductive state interaction was observed which suggests that in ewes the physiological state can override some of the physical limitations to intake imposed by increased dietary roughage concentration. INTRODUCTIONSeveral studies have shown in both ruminant and non-ruminant animals, given suitable diets, that there is a tendency for food intake to be regulated so that energy intake in the longer term meets energy demand. This regulation is responsive both to changes in energy output (Mayer, 1955) and to changes in the energy content of the diet (Owen & Ridgman, 1967). Many of these studies also demonstrate that the response is only fully effective in matching energy intake and demand over a restricted range of dietary energy content, usually at the higher end of that found in normal diets for the species. Factors such as the physical form of the diet (fineness of grinding of the constituents, for example) modify the range of operation of this intake regulatory mechanism.Although there is evidence of differences in the speed and extent of the intake response according to age and weight in pigs, for example (Owen, Ridgman & Wyllie, 1971), there is little satisfactory evidence to determine whether and to what extent increases in energy output by the animal can override what has become known as the 'physical limitation' on intake which is of particular interest in ruminant animals. Favoured hypotheses of the control of voluntary intake in * Present address: