Three phase II studies of the serotonin antagonist GR38032F were conducted. In trial 1, 20 patients given initial chemotherapy with cisplatin at doses greater than or equal to 100 mg/m2 were randomized to receive three GR38032F doses (0.18 mg/kg) on an every-2-hour or every-4-hour schedule. In trial 2, eight similar patients were given three 0.04-mg/kg doses every 2 hours. In trial 3, 12 previously treated patients receiving cisplatin at 20-25 mg/m2 on 4 or 5 consecutive days each received three daily GR38032F doses (0.15 mg/kg) every 2 hours. In trial 1, 35% of the patients had no emesis [95% confidence interval (CI), 16%-58%] and 55% had one or two emetic episodes (95% CI, 32%-76%). Results were similar between the every-2-hour and every-4-hour schedules. In trial 2, only one of eight patients (13%) had no vomiting (95% CI, 1%-50%). In trial 3, in which previously treated patients were studied, complete control ranged from 75% on day 1 to 33% on day 3. Mild sedation, headache, and transient elevations of serum SGOT (AST) were observed. No extrapyramidal symptoms occurred. A dose of 0.15-0.18 mg/kg every 2 hours for three iv doses is recommended. Further comparison and combination studies of GR38032F are warranted.