In this fully automated method for determination of 3-methylhistidine (3MH) in plasma we use precolumn derivatization with o-phthaldialdehyde and subsequent separation by HPLC. Total analysis time is 36 min, and peak areas measured vary linearly with the amount of analyte injected, over the range of 0 to 20 pmol of 3MH (R2 = 0.995), with a coefficient of variation (CV) of 1.6%. The method is reliable, accurate, inexpensive, and at least 1000-fold more sensitive than conventional ion-exchange chromatography with ninhydrin. Because of its sensitivity, the method can be used to estimate venous-arterial differences. In four human volunteers the plasma 3MH concentration varied between 4.97 and 6.08 mumol/L, and the difference between "arterialized" and femoral venous blood for 3MH varied between 0.09 and 0.47 mumol/L.