Evaluations of new emission tomography ligands are usually carried out in animals. In order to keep the animals in a restricted position during the scan session, anaesthesia is almost inevitable. In ex vivo rat studies we investigated the interference of ketamine/xylazine, zoletile mixture, isoflurane and halothane with the serotonin re-uptake site, the serotonin(2A) receptor and the dopamine re-uptake site by use of [(3)H]-(S)-citalopram, [(18)F]altanserin and [(125)I]PE2I, respectively. Ketamine/xylazine decreased the target-to-background ratio (mean +/- SD) of [(3)H]-( S)-citalopram from 1.5+/-0.19 to 0.81+/-0.19 (P<0.05), whereas isoflurane and halothane increased the ratio from 1.5+/-0.19 to 1.9+/-0.24 and 2.1+/-0.13 (P<0.05), respectively. Only with the zoletile mixture did the ratio remain unaltered. None of the tested anaesthetics affected the target-to-background ratio of [(18)F]altanserin. The [(125)I]PE2I target-to-background ratio decreased with both ketamine/xylazine (from 12.4+/-0.81 to 10.1+/-1.4, P<0.05) and isoflurane (from 12.4+/-0.81 to 9.5+/-1.1, P<0.05) treated rats, whereas treatment with zoletile mixture and halothane left the ratio unaltered. It is concluded that prior to performance of neuroreceptor radioligand studies, the possible interaction between radioligands and anaesthetics should be carefully evaluated.