The effects of gastric acidity on the bioavailability of scopolamine hydrobromide and ${ell}$-hyoscya-mine were studied in rabbits. Each drug was administered orally at dose of 0.2mg/kg to gastric acidity controlled rabbits. The plasma concentration of scopolamine and ${ell}$-hyoscyamine were determined by selected ion monitoring in GC/MSCScopolamine; m/Z= 138, ${ell}$-Hyoscyamine; m/z= 124). Rabbits with hypoacidity showed significantly higher Cp and AVC than rabbits with hyperacidity after oral administration of each drug. From the results of this experiment, it is desirable that dosage regimen of dose of scopolamine and hyoscyamine should be adjusted when the drugs should be administered with antacids in clinical practice.