Effects of various types of scorpion venom on gamma-aminobutyric acid (GABA) release were studied in an isolated synapse in the crayfish. Post-synaptic GABA-induced currents were recorded to monitor the GABA release from the pre-synaptic site. In 20 mM tetraethylammonium (TEA) chloride solution the GABA-induced cur-rents increased 71%. Exposing the preparations to Leiurus quinquestriatus hebraeus, Leiurus quinquestriatus quinquestriatus or Tityus serrulatus venom (0.1 mg/ml) increased GABA-induced currents 4-5 fold. The effect was present in the presence of tetrodotoxin (TTX) but diminished significantly when verapamil was applied. Exposing the preparations Androctonus australis or Buthus tamulus venom did not affect the GABA-induced currents. The results indicate that stimulation of the GABA release by some of the scorpion venoms may partly be due to a possible block of pre-synaptic potassium channels, but not due to an abnormal increase in sodium channel activation. (C) 2003 Elsevier Science Ltd. All rights reserved.