1. Acid secretion for each dog has reached a near maximum (100%) at the 6th samples, 90 min after the intravenous infusion of histamine (10 mu ghr-1, or approximately equal to 0.3 mghr-1). 2. 0.5 mgkg-1 Cimetidine had produced a mean inhibition of 47% on the stomach. 3. 0.1 mgkg-1 Ranitidine (D 14,951) could only inhibit a maximum of 28%, and the secretion had return to normal in just 30 min. 4. 0.025 mgkg-1 Tiotidine (D 15,104) had inhibited 53% acid secretion within 15 min of exposure. Recovery was quite similar to that of Cimetidine, at 150 min. 5. At a dosage one fifth of Cimetidine (0.1 mgkg-1) D 15,144 had depressed 35% of acid secretion at the first 15 min. The inhibition is gradually increased to about 43% (at 30 min), and was maintained for the next 105 min.