High performance liquid chromatography procedures have been developed for the assay of morphine-ondansetron and meperidine-ondansetron mixtures in 0.9% sodium chloride injection. The separation and quantitation of morphine-ondansetron were performed on an underviatized silica column at ambient temperature using a mobile phase of 60:40 v/v 0.01Mmonobasic potassium phosphate pH 4.0 - methanol at a flow rate of 1.0 mL/min with the detection set at 233 nm. The separation was achieved within 20 min. Morphine and ondansetron were linear in the 134–536 and 68–271 μg/mL ranges, respectivly. Accuracy and precision were in the range 0.04–4 and 0.2–1%, respectively, for the two analytes and the limits of detection for morphine and ondansetron were 210 and 110 ng/mL, respectively, based on a signal to noise ratio of 3 and a 20 μL injection. The separation and quantitation of the meperidine-ondansetron mixture were also achieved on an underivatized silica column at ambient temperature using a mobile phase of 60:40 v/v 0.01Mmonobasic potassium phosphate pH 4.0 - methanol at a flow rate of 1.0 mL/min with detection of the two analytes at 254 nm. The separation was achieved within 20 min. Meperidine and ondansetron were linear in the 556–3331 and 89–536 μg/mL ranges, respectively. Accuracy and precision were in the range 0.7–5.1 and 0.1–0.6%, respectively, for the two analytes and the limits of detection for meperidine and ondansetron were 1.73 μg/mL and 47 ng/mL, respectively, based on a signal to noise ratio of 3 and a 20 μL injection.