Concentration of oxygen, methane, and hydrogen were measured in intracolonic gas samples aspirated through the colonoscope at the time of colonoscopy from 46 patients. Of the above patients 20 prepared either with mannitol (n = 10) or with castor oil (n = 10) had the instrument passed to the caecum without air insufflation or suction. After mannitol, mean intracolonic hydrogen concentration (4.07%) was significantly higher (p less than 0.001) than after castor oil (0.51%). Mean oxygen and methane concentrations were approximately similar. Potentially explosive concentrations of hydrogen (greater than 4.1%) and or methane (greater than 5%) were present in 6/10 patients given mannitol and 2/10 patients given castor oil. Nevertheless only one patient from each group had coexisting oxygen concentrations of more than 5% producing thus a combustile mixture. Routine colonoscopy (using air insufflation and suction) was performed in 26 patients prepared with mannitol. Mean intracolonic hydrogen and methane was 0.63% and 0.88% respectively. The highest recorded concentration of hydrogen was 2.6%, and of methane 2.1%, while all patients had oxygen concentrations of more than 5%. It is suggested, therefore, that routine insufflation and suction before colonoscopic electrosurgical polypectomy should result in safe levels of these gases. The remote possibility of pockets of undiluted gas in explosive concentration, however, indicates the use of an inert gas such as carbon dioxide if mannitol preparation is used before electrosurgery.