Analysis of the respiratory ¹⁴CO₂ of growing chicks injected intraperitoneally with [1–¹⁴C] or [U-¹⁴C]glycine or valine has shown the presence of a delayed ‘slow ¹⁴CO₂’ exponential component. The half-lives, about 3 h, are independent of the nature of the amino acid and of the age (1–3 weeks) of the animal. As supported by investigations in cycloheximide-treated chicks of the oxidative catabolism of glycine, valine and acetate, slow ¹⁴CO₂ is significantly associated with the degradation of short-lived proteins.