The effects of alpha 2-adrenoceptor and GABA receptor agonists on citric acid-induced cough and increased tidal volume were investigated in conscious guinea pigs. Inhalation of low doses of B-HT 920 (5-allyl-2-amino 5,6,7,8-tetrahydro-4H-thiazolo[4,5-d]azepine dihydrochloride), and xylazine significantly inhibited citric acid-induced cough and tidal volume increases. Intraperitoneal administration of higher doses of B-HT 920 than those given by aerosol were ineffective. The inhibitory effects of B-HT 920 were antagonised by prior intraperitoneal administration of yohimbine, but not atropine. Inhalation of GABA or baclofen inhibited tidal volume increases, but had no effect on cough. Inhaled alpha 2-adrenoceptor or GABA agonists had no effect on the reduced respiratory rate after citric acid inhalation. It is concluded that alpha 2-adrenoceptor agonists inhibit cough via a mechanism which may not be related to their ability to reduce citric acid-induced tidal volume increases, since GABA and baclofen inhibited tidal volume increases but not cough. We suggest that alpha 2-adrenoceptor agonists may have therapeutic potential in the treatment of cough.