To assess whether there is rebound of gastric acidity after a short-term course of H2-receptor antagonists, we performed a 24-h in situ pH monitoring on 17 duodenal ulcer patients before, during, and after completing 4 wks' treatment with nizatidine 300 mg at bedtime (2200 h). Three around-the-clock continuous intragastric pH-metries were carried out the day before beginning treatment, at the end of the nizatidine course, and 67 h after the last dose of the drug was given. Nizatidine produced mean pH values that were significantly higher (p less than 0.001) than those of the pretreatment profile during both the whole 24-h period and the nighttime (2400-0759), and this confirms that it effectively inhibited gastric acidity. Conversely, there was no significant difference between the acidity indexes of pH profiles obtained before dosing and after discontinuing treatment in any of the time intervals considered. Therefore, it appears that a short term course of nizatidine therapy does not cause rebound hyperacidity in duodenal ulcer patients.