The measurement of skeletal muscle protein fractional synthetic rate using an infusion of (1‐¹³C)leucine and measuring the isotopic abundance of the tracer in skeletal muscle protein by preparative gas chromatography (GC)/ninhydrin isotope ratio mass spectrometry (IRMS) is laborious and subject to errors owing to contamination by ¹²C. The purpose of this study was to compare muscle (¹³C)leucine enrichment measured with the conventional preparative GC/ninhydrin IRMS approach to a new, continuous‐flow technique using capillary GC/combustion IRMS. Quadriceps muscles were removed from four Sprague‐Dawley rats after each was infused at a different rate with (1‐¹³C)leucine for 6–8 h. Muscle leucine enrichment (at.% excess) measured by both methods differed by less than 4%, except at low (¹³C)leucine enrichments (<0.03 at.% excess). In addition, capillary GC/combustion IRMS was used to assess muscle (¹³C)leucine enrichment and fractional muscle protein synthesis rate in ten normal young men and women infused with (1,2‐¹³C₂)leucine for 12–14 h. This approach reduced the variability of the isotope abundance measure and gave estimates of muscle protein synthesis rate (0.050 ± 0.011% h⁻¹ (mean ± SEM); range = 0.023‐0.147% h⁻¹) that agree with published values determined using the standard analytical approach. The measurement of (¹³C)leucine enrichment from skeletal muscle protein by capillary GC/combustion IRMS provides a simple, acceptable and practical alternative to preparative GC/ninhydrin IRMS.