The proboscis extension response (PER) which can be elicited in bees by stimulating one antenna with water vapour, was used to quantify the effects of dopamine, serotonin (5-HT) and putative receptor ligands in the mushroom body of the bee. The drugs were microinjected into the alpha-lobe of the mushroom body in one brain hemisphere. Injection of dopamine reduces the water vapour-elicited PER significantly. The effects of dopamine are limited to the treated side. Injection of 5-HT has similar effects to dopamine. The effects of 5-HT are apparent on the treated and partly also on the contralateral side. Significant effects for dopamine on the treated side were found when the concentration in the injected drop was 10(-7) M. For 5-HT significant effects on the treated side were apparent for concentrations of 10(-8) M. Putative dopamine and 5-HT receptor ligands were injected alone and coinjected with the amines. Two ligands with dopamine-antagonistic effects were found: buspirone > spiperone, while lisuride, sulpiride, chlorpromazine, SCH 23390, butaclamol and haloperidol had no dopamine-antagonistic effects. All tested putative 5-HT receptor ligands had significant 5-HT-antagonistic effects: butaclamol > methysergide > lisuride > cyproheptadine > SCH 23390. Good correlations between the behavioural data and in vitro radioligand binding studies were found for 5-HT receptor ligands, while there exist only partial correlations for dopamine receptor ligands.