Following intravenous injection, technetium-99m (99Tcm) is selectively taken up by the stomach, and this uptake, which is stimulated by pentagastrin, may be quantified using a gamma camera. Using this property of 99Tcm as a means of assessing gastric function, we found that the mean rate of 99Tcm uptake was significantly greater in 24 duodenal ulcer patients than in 10 healthy controls (P less than 0.01). Scanning values did not correlate with BAO of MAO which were measured on a separate day. Both vagotomy (n = 10) and cimetidine infusion (n = 5) significantly reduced the rate of uptake of 99Tcm (P less than 0.01). Thus, although 99Tcm reflects a function of the stomach which is similar to acid secretion, scanning did not give a good estimate of acid secretion in this group of subjects.