The role of gamma-hydroxybutyric acid (GHBA) and gamma-aminobutyric acid (GABA) in the mechanisms of respiratory arrest after systemic serotonin administration has been studied in anaesthetized outbred male rats. Under normal conditions, serotonin (5-HT) administration (20 - 60 microg/kg, i.v.) leads to specific changes in the respiratory pattern, whereby a short (approximately 5 sec) increase in the respiratory rate is followed by a longer (up to approximately 20 sec) respiratory arrest. Pretreatment with Li-GHBA salt (750 mg/kg, i.v.) did not significantly change the first stage of the respiratory response to serotonin administration, but excluded or significantly reduced the stage of respiratory arrest. The effect of bilateral vagotomy on the respiratory pattern upon serotonin administration was very similar to the action of GHBA. It is suggested that GHBA as a neuromodulator probably participates in the mechanisms of respiratory control after serotonin administration.