Two forms of bowel preparation for colonoscopy were compared--19 patients were given 5 per cent mannitol solution orally while a further 19 were infused with isotonic saline via a nasogastric tube. Both methods proved equally acceptable to the patients and endoscopists. Saline led to a rise in body weight (+0.75 +/- 0.35 kg) and blood pressure (+7.5 +/- 2.8 mmHg) while mannitol caused a significant fall in both body weight (-0.74 +/- 0.28 kg) and blood pressure (-3.8 +/- 2.9 mmHg). Plasma volume measurements were carried out on 17 patients; 8 receiving saline showed a rise (+0.22 +/- 0.08 l) while 9 taking mannitol experienced a fall (-0.17 +/- 0.08 l). Twelve further subjects prepared with a combination 10 per cent mannitol and isotonic saline showed a small fall in plasma volume (-0.08 +/- 0.05 l). Mannitol proved an acceptable preparation for colonoscopies but a fall in body weight, blood pressure and plasma volume posed a small risk to the elderly subject and in view of the known risks of mannitol during diathermy this form of preparation was not considered a suitable alternative to isotonic saline.