In conscious rats with normal arterial blood pressure or with spontaneous or DOC induced hypertension, effects of drugs increasing (cycloserine, ethanolamine-o-sulphate, piracetam, chlordesmethyldiazepam) or depressing (bicuculline, Ro 15-1788, PK 11195) GABAergic reactivity were evaluated on the arterial hypotension and sinus bradycardia induced by clonidine. Clonidine was administered orally by gastric gavage (0.1-1 mg/kg). Pretreatment with cycloserine, ethanolamine-o-sulphate, piracetam or chlordesmethyldiazepam increased the hypotension and sinus bradycardia induced by clonidine. On the contrary, pretreatment with bicuculline or Ro 15-1788 reduced the cardiovascular effects of clonidine. These results suggest that the GABAergic system is involved in cardiovascular effects of clonidine in normotensive and hypertensive rats.