In a double-blind, cross-over study, we have investigated the effect of oral diazepam 20 mg and placebo on the sensitivity of upper airway reflexes in 10 male volunteers (aged 25-35 yr). Upper airway reflex sensitivity (UARS) was assessed using small concentrations of ammonia vapour as a stimulus to upper airway receptors. A threshold concentration of ammonia, at which reflex glottic closure occurred in response to the ammonia stimulus, was used as a measure of UARS. With diazepam, there was significant depression of UARS from 30 to 150 min after administration.