Voltammetric determination of tryptophan (Trp) and serotonin (5-HT) was successfully conducted. To do this, we analyzed the redox peaks of their oxidation products at glassy carbon paste electrodes (GCPEs) in phosphate buffer solutions in the presence of interfering substances. The oxidation products of Trp and 5-HT provided redox couples at more negative potentials than for that of the interfering substance: fivefold tyrosine (Tyr) did not hinder the determination of Trp, and 200-fold uric acid (UA) and acetaminophen (APAP) did not hinder the determination of 5-HT.