A double blind study comparing intravenous pethidine and meptazinol has been performed to establish the efficacy and safety of meptazinol as an analgesic agent in colonoscopy. Twenty two patients received pethidine and 23 patients received meptazinol and no difference in analgesic effect or sedative effect could be demonstrated either by observer or patient assessment using a visual analogue scale. A group of 10 patients in the pethidine group and 9 in the meptazinol group had continuous recording of electrocardiogram, pulse rate and blood pressure throughout the procedure. Significant falls in both systolic and diastolic blood pressure were recorded in the pethidine group but not the meptazinol group. Benign cardiac arrhythmias were recorded in both groups before and after the administration of premedicant drug and 1 patient in each group had transient ST depression. Side effects were recorded with equal frequency in each group except for vomiting which occurred in 5 of 23 meptazinol patients but none of the pethidine patients. Meptazinol is an effective analgesic drug in colonoscopy which produces less cardiovascular depression than pethidine and thus may be useful in selected patients especially the elderly or those with known cardiovascular disease.