A rapid and accurate method is described for the determination of prolyl peptides in urine, with specific reference to the dipeptide prolylhydroxyproline, and free hydroxyproline and proline. Free amino acids and peptides were isolated from urine on cation-exchange minicolumns, and free imino acids and prolyl-N-terminal peptides were selectively derivatized with 4-chloro-7-nitrobenzofurazan, after reaction of amino acids and N-terminal aminoacyl peptides with o-phthalaldehyde. The highly fluorescent adducts of imino acids and prolyl peptides were separated on a Spherisorb ODS 2 column by isocratic elution for 12 min using as mobile phase 17.5 mM aqueous trifluoracetic acid solution containing 12.5% acetonitrile (eluent A), followed by gradient elution from eluent A to 40% of 17.5 mM aqueous trifluoroacetic acid solution containing 80% acetonitrile in 20 min. Analytes of interest, in particular the dipeptide prolylhydroxyproline, can be easily quantified by fluorimetric detection (epsilon ex = 470 nm, epsilon em = 530 nm) without interference from primary amino-containing compounds.