Background:  Two doses of sodium phosphate are recommended for colonic cleansing before colonoscopy. Patients need information about the onset and duration of activity of the preparation. This study was designed to develop practical prescribing information to patients to reduce anxiety concerning bowel accidents. Methods:  Two hundred patients were requested to take a liquid diet the day before colonoscopy, to take 45 mL sodium phosphate with water or juice the evening before and the morning of the procedure, and to complete a questionnaire. Results:  Bowel activity began 1.7 hours (range 0.5 to 13) after the first dose and continued for 4.6 (1 to 14) hours. For the second dose the time to onset was 0.7 hours (0.25 to 4), and bowel activity continued for 2.9 hours (0.5 to 6.5). Bowel activity was completed 4 hours after ingestion of the second dose in 83% of patients and within 5 hours in 87%. Three of 200 patients reported the need for bowel evacuation during transit. Conclusions:  The activity from the second dose of sodium phosphate starts and ends sooner than that from the first dose. When advising patients on when to take the second dose of sodium phosphate, 4 hours for most and 5 hours for those with long commutes, should provide adequate time for almost all patients to avoid bowel activity during travel. (Gastrointest Endosc 1999;50:811-3.)