MECLONAZEPAM (3-methylclonazepam) has been found to be orally effective at high doses against all stages of schistosomiasis. Animals studies have shown it to have a high therapeutic index and a profile of behavioural activity typical of the benzodiazepines. The effects of single oral doses of meclonazepam, 1, 2 and 4 mg on central arousal, psychomotor performance and subjective mood were studied in two double-blind placebo controlled studies in healthy volunteers. In doses exceeding 1 mg, meclonazepam caused marked dose-related impairment in cognitive and psychomotor functions as well as shifts in mood reflecting sedation and ataxia. These effects were most prominent in the first 3 h after administration, with moderate sedation still present 6 h after the 4 mg dose. The implications of these findings for the use of benzodiazepine agents in the treatment of schistosomiasis are discussed.