In the context of the questions under discussion here, three findings from this study should be briefly emphasised: (1) Neurophysiological parameters investigated in healthy subjects receiving single doses of the two enantiomers of oxaprotiline in an acute experiment, performed in the sleep laboratory under defined conditions, revealed marked qualitative and quantitative differences between the effects of these enantiomers. (2) Since the two enantiomers are practically devoid of any anticholinergic or anti-serotonergic effect and display similar antihistaminic and sedative-antiaggressive activity, it is reasonable to assume that the differences in their effects on the sleep parameters measured are attributable to the difference in their pharmacodynamic profiles with respect to inhibition of noradrenaline uptake. (3) REM sleep parameters correlate significantly with the effect of the d-(+)-enantiomer, i.e. with its selective effect on inhibition of noradrenaline uptake.