Organotypic primary cell cultures of fetal rat brain were used as a model system to study the effect of COMT inhibitors on the cerebral metabolic conversions of fluoro-DOPA enantiomers. The selective COMT inhibitors OR 486 and CGP 28014 were used in conjunction with 5F-L-DOPA, 6F-L-DOPA and 6F-D-DOPA as substrates. Methylation can be clearly reduced by application of OR 486 at nanomolar level, without inhibition of AADC and MAO. The uptake of the substrate is unchanged. CGP 28014, already known to be active only in vivo, has no influence on the metabolic conversion rates of the fluoro-DOPA isomers. These results show that use of this culture system allows statement concerning the in vitro activity of COMT inhibitors. It has not been possible to show an increase of absolute levels of decarboxylation products due to inhibition of COMT, however, but the reduction in levels of methylated product itself may have significance for PET studies of the human brain.