An incomplete factorial experiment was conducted to determine the effect of level and frequency of feeding of a protein-rich supplement on the growth and whole-body protein metabolism of young sheep fed a medium quality roughage diet. Cottonseed meal (CSM) was used as the protein supplement and provided at 0, 0.2 or 0.4% liveweight per day at a frequency of 1 or 3 times each week and chopped oaten (0.95) and lucerne (0.05) hay was the roughage. Growth rate more than doubled (P < 0.01) following provision of CSM but there was no advantage of feeding CSM at the highest level. Frequency of feeding CSM did not alter growth rate. Intake of hay was little affected by CSM and as a consequence the food conversion ratio declined (P < 0.01) favourably from 22 : 1 (nil CSM) to 9 : 1 as a result of supplementation. The rate of whole-body protein synthesis increased (P < 0.01) in response to the highest level of CSM with no apparent change in protein degradation, underpinning an increase (P < 0.01) in protein retention. These results highlight the role of protein supplements for promoting growth of young sheep on roughage diets and indicate that these supplements need to be provided only once a week.