Following a double-blind, four-way crossover design, 32 healthy volunteers (20 males and 12 females) each consumed lactose placebo, or 80, 120 or 160 mg quinine HCl daily for 21 days. Before dosing and at regular intervals during dosing, blood and urine samples were collected and analysed for quinine HCl. Electrocardiography, heart rate, blood pressure, audiometry, peripheral field, funduscopy, colour vision, visual acuity, electronystagmography (ENG) and test for optokinetic nystagmus were all evaluated before dosing and at selected times during dosing. The results showed that daily consumption of up to 80 mg quinine HCl did not significantly alter physiological, ophthalmic or audiometric responses. ENG determination showed that 12.5% of volunteers given lactose placebo or 80 mg quinine HCl exhibited at least one transitory period of ocular motor oscillations. This phenomenon was observed in 18.8% (P < 0.05) of volunteers with a daily intake of 120 mg quinine HCl or more. However, there was not a significant dose-related correlation between nystagmus and daily intake of quinine HCl. Five volunteers consuming lactose placebo displayed an aberrant ocular flutter that decreased significantly (P < 0.05) as the daily intake of quinine HCl increased. One volunteer showed a change in perception of red/green colour vision after taking 160 mg quinine HCl for 21 days. This study demonstrated that the no-untoward-effect level of quinine HCl is at least 80 mg/day.