The effects of several alpha 2-adrenoceptor agonists and antagonists were examined on gastric acid secretion from reserpinized rats undergoing electrical stimulation of the left vagus nerve. Both detomidine and oxymetazoline inhibited vagal acid hypersecretion, their effects being fully prevented by idazoxan, 7,8-(methylenedioxy)-14-alpha-hydroxyalloberbane (CH 38083), or 2-(2-methoxy-1,4-benzodioxan-2-yl)-2-imidazoline (RX 821002), and partly antagonized by yohimbine or rauwolscine. 2-(2,4-(O-methoxy-phenyl)-piperazin-1-yl)-ethyl-4,4-dimethyl-1,3-( 2H,4H)-isoquinolindione (ARC 239) did not affect the antisecretory action of the two agonists, while prazosin at the highest dose was partly effective only against detomidine. Atropine markedly reduced vagally evoked acid output. It is suggested that acid secretion induced by vagal cholinergic stimulation is modulated by alpha 2A-like adrenoceptor subtypes.