The efficiency of dietary protein utilisation in broiler chickens was studied by conventional balance techniques and by digestion and metabolism of ¹⁵N-labelled protein in the diet. Between 16 and 30 days of age, male chickens tended to be heavier and to ingest more feed than female chickens: however, female chickens tended to absorb and utilize dietary protein more efficiently than male chickens. Whole-body synthesis rate did not differ between sexes and was 6.7-7.5 times the protein absorption rate. Only about 10% of the protein synthesized was deposited in tissue gain whereas 90% was degraded. Fractional protein synthesis rates were determined from the decline in enrichment in small intestinal, liver and breast muscle samples taken sequentially from birds that had earlier ingested ¹⁵N-labelled feed. FSRs declined with age but did not differ between sexes and were 44-52, 23-29 and 15-18 %/d, respectively.