Abstract.We have used the forearm model to study protein metabolism in six normal healthy subjects in the fed state using L‐[1 –13C,15N]‐leucine as the substrate tracer.Deep venous and arterialized venous blood samples from the forearm were collected at 10‐min intervals 2±5 h into a primed‐continuous infusion of the dilabelled tracer. Arterialized venous blood was obtained using a ‘hot‐box’ technique and forearm blood flow was measured by mercury strain‐gauge plethysmography.The concentration and isotope enrichment of leucine and its metabolites, α‐ketoisocaproic acid and CO2, in deep venous and arterialized venous blood were measured by gas chromatography‐mass spectrometry and isotope ratio‐mass spectrometry.The rates of leucine deamination and reamination were 388 ± 24 (mean ± SEM) and 330 ± 23 nmol (100 ml)‐1min‐1respectively, whilst protein synthesis and breakdown rates were 127 ± 11 and 87 ± 10 nmol (100 ml)‐1min‐1respectively across the forearm in the fed state. We have demonstrated that the use of doubly labelled leucine as tracer and application of the mathematical model developed in this study, permits the comprehensive quantification of leucin