Methods and compositions are disclosed utilizing the optically pure R(+) isomer of ondansetron. This compound is a potent drug for the treatment of nausea and vomiting associated with chemotherapy and radiation therapy, while avoiding the concomitant liability of adverse effects associated with the racemic mixture of ondansetron. The R(+) isomer of ondansetron is also useful for the treatment of behavioral disorders such as mood anxiety and schizophrenia, and such other conditions as may be related to R(+) ondansetron's activity as a competitive antagonist of serotonin receptor subtype 5-HT.sub.3 such as disorders of gastrointestinal motility, depression, migraine, and as an aid for alcohol withdrawal, nicotine withdrawal, and drug (benzodiazepine et al.) withdrawal, without the concomitant liability of adverse effects associated with the racemic mixture of ondansetron. Furthermore, the R(+) isomer of ondansetron is also useful for the treatment of cognitive disorders such as dementia or age-associated memory impairment, while avoiding the concomitant liability of adverse effects associated with the racemic mixture of ondansetron.