BACKGROUND Nausea and vomiting are well-known gastrointestinal complications in chronic renal failure and are frequent indications for the commencement of dialysis. Although the administration of antiemetic drugs (metoclopramide and, recently, ondansetron) is usually mentioned, there are scanty data on their effects. METHODS A double-blind crossover study was done in 10 uremic patients. All the patients were uremic and suffered from nausea and vomiting. The drugs were randomly administered intravenously (either metoclopramide 10 mg or ondansetron 8 mg) 2 h after blood drawing for laboratory tests either on the 1st or on the 3rd study day at the same time. The outcomes were scored after 24 h of follow-up by (1) one of us (D.P.; 1-3 points: 1 = no effect; 2 = moderate effect - decreased frequency of vomiting, and 3 = good effect - no vomiting), and (2) by the patients (1-5 points). RESULTS The results obtained showed that ondansetron was more effective in controlling nausea and vomiting than metoclopramide, either objectively (2.80 +/- 0.422 vs. 1.40 +/- 0.699, p < 0.005) or subjectively (4.10 +/- 0.738 vs. 2.10 +/- 0.994, p < 0.005). CONCLUSIONS We conclude that at the dosage level studied ondansetron is about twice as effective as metoclopramide in the symptomatic relief of uremia-induced nausea and vomiting.