We demonstrated pharmacokinetic characteristics of cisapride in elderly patients (85+/-6 years) who had suffered from constipation based on intestinal motility dysfunction. After a single p.o. dose of 2.5 mg cisapride tablet on the first day, plasma unchanged cisapride concentrations were detected at 9 points within 24 hours, using high-performance liquid chromatography method. Subsequently, the patients were treated with cisapride 3 times daily during next 26 days (from the day 2 to the day 27). On the 28th day, a cisapride tablet was given once in the morning and blood samples were obtained with the same procedure as that on day 1. The obtained elimination half-life averaged 21.8+/-10.6 hours, which seems to be considerably longer than those previously reported. The area under the time-concentration curve (AUC) on the day 28 was 2.7 times larger than AUC on the day 1, suggesting that plasma cisapride concentrations can become higher than expected by physicians when given 3 times daily to extremely elderly patients. Because cisapride sometimes induces fatal ventricular arrhythmias and sudden death in a concentration-dependent manner, this medicine should be administered once or twice per day to such patients.