Roxatidine acetate hydrochloride capsule is slowly absorbed from the gastrointestinal tract, and its acid suppressive effect on the stomach is long-lasting compared with other H2-blockers. The reduction of gastric juice in perioperative period is considered advantageous for patients not only because it decreases the risk for aspiration pneumonia but also because it reduces the risk of bronchial spasm induced by gastroesophageal reflux of acidic gastric content. The effects of single oral administration of roxatidine acetate hydrochloride 150 mg at night before the operation on the volume and pH of gastric juice were investigated during anesthesia using two types of anesthetic agents (isoflurane and propofol) in 93 patients of three age groups (group Y: age 20-40, group M: age 41-64, group O: age 65 <). The effect of roxatidine on reduction of gastric juice was found at the time of anesthetic induction and 2 hours after the induction in any age group with either anesthetic agent. The serum concentration of roxatidine at the time of induction was much higher in group O. The value of residual concentration of roxatidine 20 hours after oral intake was estimated from the intraoperative measurements of serum concentration. The results suggest that single administration at night before the operation is sufficient for the oldest group, but an additive dose is recommended for the younger groups.