We have examined the effects of exogenous and intrasynaptosomal histamine on the GABA transport in rat brain synaptosomes. Exogenous histamine had no effect on GABA uptake, but inhibited veratridine-evoked GABA release. GABA uptake was susceptible to the modification of histamine synthesis in synaptosomes. The increase of synaptosomal histamine level by incubation under conditions favouring histamine synthesis from 2.6 to 5.2 pmole/mg protein, decreased the GABA uptake by about 75%. As demonstrated in experiments with an inhibitor of histidine decarboxylase--semicarbazide, this effect comprised 15% inhibition evoked by the histamine precursor--histidine, whereas the remaining 60% inhibition could be ascribed to the effect of newly synthesized histamine.