Since the development of clozapine the investigation of atypical neuro
leptic compounds has become increasingly relevant. Compared with class
ic neuroleptics they are distinguished by either fewer or absent (cloz
apine) extrapyramidal side effects, some of them also by lower increas
es of serum prolactin concentrations. Pharmacologically they are a gro
up of heterogeneous substances. At the level of transmitter systems a
high 5HT2/D2-ratio is regarded as the best criterion to distinguish be
tween atypical and classic neuroleptics. Further differences involve:
the preferred effects of atypical neuroleptics on mesolimbic D2 recept
ors compared to striatal dopaminergic neurotransmission; a higher pote
ncy of some atypical neuroleptics to antagonize D1-receptors; the incr
ease of serum corticosterone concentrations by some of the atypical ne
uroleptics. The possible effects of atypical neuroleptics and their cl
inical relevance are discussed.