Whole body protein turnover was measured in 11 patients before and after elective orthopaedic operations by giving 15N-glycine orally every 4 hours for 32 hours. The patients were maintained throughout on a constant protein intake. In 2 control subjects a comparison was made between intermittent dosage and continuous infusion of 15N-giycinefor the estimation of total protein turnover. With intermittent dosage the 15N abundance in urinary urea reached a constant level after about 24 hours. Rates of total protein synthesis and breakdown were calculated from the 15N abundance at the plateau level. After surgery there was a moderate increase in urinary N output. The apparent N balance (intake—urinary N) was −0·52 ± 1.31 g/d (mean ± s.d.) before operation and −7·51 ± 4.5 g/d after operation. The rate of protein synthesis fell from 3·83 ± 0·73g kg−1 d−1 before operation to 2·94 ± 0·83 g kg−1 d−1 after operation. This difference is statistically significant (0·05 > P > 0·01). There was no significant change in the rate of protein breakdown. The possibility remains that a block in protein synthesis, probably mainly in muscle, may be partly responsible for the so-called atabolicloss of nitrogen after injury, but this has not been proved.