1. Thirty Suffolk × Half bred lambs were slaughtered at the following ages: two twin lambs at birth and two singles and two twins at 1, 2, 3, 4, 6, 8 and 16 weeks of age.
2. The following weights were recorded: live-weight immediately before slaughter; and carcass, head, skin, feet, alimentary tract, heart, liver, kidneys, lungs and trachea, and blood immediately afterwards.
3. The alimentary tract was emptied and weighed in four separate parts; reticulo-rumen, omasum-abomasum, small intestine, large intestine.
4. The volumes of the reticulo-rumen and the omasum-abomasum were measured by immersing in water and filling the organs with water to 2 cm. pressure.
5. The in vitro digestive efficiency of rumen liquor from lambs of 1, 2, 3 and 4 weeks of age was assessed.
6. Empty body weight was considered to be valuable in comparing animals of different ages or from different feeding regimes or at different times of the year because variations in gut ‘fill’ were eliminated.
7. There were no differences between singles and twins in the relationship of the fresh weights of the parts of the body to empty body weight, except that development of the liver and the blood was rather slower for singles.
8. Little evidence was found of a difference in rate of development of the alimentary tract between singles an d twins, although the log an d square root transformation suggested a possible difference in reticulo-rumen size in favour of twins, significant at the 5% level.