Responses in lamb birthweight to supplements of dietary protein in late pregnancy usually arise from increases in the quantity of protein reaching the abomasum and subsequently absorbed in the small intestine. This principle is now embodied in new systems for estimating the protein needs of ruminants (see for example ARC 1980) and quantitative data are now being accumulated on dietary factors that influence protein flow to the small intestine. Although dietary factors are extremely important in determining the amounts of protein that escape from the rumen undegraded, there is evidence that the physiological state of the animal, through its effect on the retention time of food in the rumen, can also influence the amount of protein reaching the abomasum. For example, Thompson, Robinson and McHattie (1978) showed that when the energy and protein intakes of ewes remained constant there was a shorter retention time of food in the rumen and more non-ammonia nitrogen reaching the abomasum in late pregnancy than in early lactation or post weaning. The present study demonstrates a similar phenomenon following shearing.