Abstract: A method of RPHPLC for the simultaneous determination of 3 amino acids and 3 modified nucleosides in pleural effusion was proposed. The sample was extracted with methanol. After centrifugation, the supernatant obtained was use for HPLC analysis using Waters sunfire C18 column as stationary phase, and mixtures of methanol and 10 mmol·L-1 ammonium acetate solution (pH 4.6) in different ratios as mobile phase in the gradient elution. UV-detection at 278 nm was adopted in the determination for tyrosine and tryptophan, 257 nm for 2′-deoxyguanosine, inosine and guanosine, and 215 nm for phenylanine. Linearity ranges were found between 2-2 000 μmol·L-1 for phenylanine and 1-1 000 μmol·L-1 for the other 5 compounds. Values of detection limit (3S/N) of the method found were in the range of 0.008 3-0.50 μmol·L-1. Values of recovery found by standard addition method were in the range of 85.8%-106%.