In two consecutive, randomized, double-blind studies the effect of ondansetron on the time to induction of anaesthesia with propofol and, subsequently, thiopentone was assessed. In each study 40 patients received either ondansetron 8 mg or placebo immediately before induction of anaesthesia with a standardized dose of propofol (2.5 mg kg−1) or thiopentone (5 mg kg−1). Times to induction of anaesthesia were determined by assessing loss of verbal response, motor power and eyelash reflex. There was no difference in either study in times to induction of anaesthesia between immediate pre-treatment with ondansetron or placebo. Side effects were minor and of similar incidence in the ondansetron and placebo groups.