A high-performance liquid chromatographic method was developed for the determination of citrulline and homocitrulline using a post-column colorimetric reaction with o-phthaladehyde and N-(1-naphthyl)-ethylenediamine. Citrulline and homocitrulline were determined with no interferences from protein amino acids. The results show that the level of citrulline in the plasma of patients with uremia on intermittent hemodialysis is higher than that in healthy human plasma, and that homocitrulline is excreted into the urine of healthy adults.