Effects of baclofen, a GABAB agonist, on gastric acid secretion were studied from a viewpoint that baclofen may stimulate or inhibit gastric acid secretion depending on the basal secretory activity in conscious or anesthetized rats. In conscious pylorus-ligated rats, baclofen (2-16 mg/kg, s.c.) depressed acid secretion, while in urethane-anesthetized pylorus-ligated rats, baclofen (4-16 mg/kg, s.c.) stimulated acid secretion. Basal acid secretion was much higher in the conscious rats than in the urethane-anesthetized rats. When gastric secretion was measured by the stomach-perfusion method, baclofen (s.c.) significantly increased acid at 2-4 mg/kg in urethane-anesthetized rats but decreased acid at 0.5-2 mg/kg in pentobarbital- and alpha-chloralose-anesthetized rats. Basal acid secretion was markedly higher in the latter rats than in the former. Moreover, baclofen did not affect acid secretion elicited by peripheral vagal stimulation in alpha-chloralose-anesthetized rats. These results suggest that acid secretion is centrally stimulated by baclofen in rats with low basal secretory activity, and vice versa, and moreover, these dual effects are closely associated with tone of the central vagal center.