1. The present study sought to determine the possible existence of a pool of proteins which turn over with life-time kinetics. The pattern of enrichment of ammonia and urea in hourly samples of urine was determined in normal adults to whom oral doses of [15N]glycine were given hourly for 36 h. The subjects received hourly meals throughout, and in six the study commenced at 06.00 hours, in five at 12.00 hours and in two at 18.00 h.
2. A plateau level of enrichment was achieved in urinary ammonia within 4–6 h. Regardless of the time at which the study started this plateau was held until about midnight, at which time there was an increase in enrichment, with a second higher plateau 5–6 h later. The second plateau was held to the end of the study. For urinary urea the rate of rise in enrichment was slower and smoother, because of the slow turnover of the urea pool.
3. Protein synthesis, derived from the first ammonia plateau, 179 mg h−1 kg−1, was significantly higher than that derived from the second plateau, 118 mg h−1 kg−1. Using the plateau in urea towards the end of the 36 h, the estimate of protein synthesis was 153 mg h−1 kg−1.
4. The results are considered to provide evidence of a pool of proteins for which degradation takes place in harmony with a circadian rhythm.