In non-stimulated rabbit gastric glands, acetylsalicylic acid (10-500 µM) and indomethacin (3-300 µM) did not significantly modify the basal rate of acid secretion, while diclofenac and piroxicam (10-1000 µM, each) caused a marked and dose-dependent inhibitory effect (EC₅₀ =138 and 280 µM, respectively). In gastric glands stimulated by histamine (100 µM), diclofenac also reduced the rate of acid formation in a dose-dependent manner. In contrast, acetylsalicylic acid, indomethacin and piroxicam exerted a biphasic effect; thus, low concentrations (3-100 µM) of these three agents significantly increased the rate of histamine-stimulated acid secretion (10-20% over the corresponding control value), by a cyclic AMP-independent mechanism, while higher concentrations reduced the rate of acid formation. With respect to the underlying biochemical mechanisms which could mediate the inhibitory effects of NSAIDs on gastric acid formation, it was observed that both diclofenac and piroxicam, but not acetylsalicylic acid or indomethacin, decreased the glandular content of ATP, inhibited the hydrolytic activity of gastric gland microsomal H⁺,K⁺-ATPase and reduced the rate of H⁺,K⁺-ATPase-dependent proton transport across microsomal membranes, in a dose-dependent manner. Furthermore, diclofenac and piroxicam also significantly increased the passive permeability to protons of microsomal membranes. In conclusion, our work shows that diclofenac and piroxicam cause a significant reduction in the rate of basal and histamine-stimulated acid formation in isolated rabbit gastric glands, at concentrations that can be attained in the gastric lumen of patients treated with these drugs. The mechanisms involved in these inhibitory effects appear to be multifocal and include different steps of the stimulus-secretion coupling.