Levodropropizine is the l-isomer of dropropizine, a racemic drug widely used as a cough suppressant. Compared with the racemate, levodropropizine retains equal antitussive activity but exhibits considerably lower central nervous system (CNS) depressant effects in animal models. In order to assess whether the same differential pharmacodynamic profile also applies to man, a double-blind placebo-controlled study was carried out to investigate the effects of single oral doses (60 and 120 mg) of levodropropizine and dropropizine on subjective alertness (scored on visual analogue scales), general tolerability and psychomotor function tests (cancellation, tapping, choice reaction times and critical flicker fusion frequency) in ten normal volunteers. Treatments were administered in random sequence at intervals of at least one week, evaluation procedures being carried out at times 0, 1, 2, 3, 4, 6 and 8 h after dosing. Following intake of a 60 mg levodropizine dose, subjective effects and objective estimates of psychomotor function were superimposable to those recorded after placebo. There was a trend for 60 mg dropropizine and 120 mg levodropropizine to produce detrimental effects at occasional evaluations, although the changes associated with these treatments could not be differentiated from placebo on the basis of most subjective scores and psychomotor function tests. Conversely, administration of 120 mg dropropizine was consistently associated with subjective CNS impairment and with reduced performance (compared to baseline) in recognition time, critical flicker fusion thresholds and possibly tapping rate, for up to three hours after dosing. These data are consistent with evidence that racemic dropropizine adversely affects central nervous system function to a greater extent compared with the levo-isomer.