The effect of duodenal ulcer healing on the acid secretory responses to modified sham feeding and maximal pentagastrin stimulation has been studied in 17 patients treated successfully with ranitidine (n = 9) and sucralfate (n = 8). Parietal cell sensitivity was calculated as the ratio of the modified sham feeding response to the peak pentagastrin response, expressed as a percentage. Ulcer healing after sucralfate therapy resulted in significant falls in modified sham feeding stimulated acid output (P less than 0.02), from 9.4 (1.8-17.0) (median + range) to 3.7 (0.2-9.4) mmol/h; in peak acid output (P less than 0.05) from 42.8 (23.0-61.4) to 27.7 (7.2-51.0) mmol/h; and in the parietal cell sensitivity (P less than 0.05) from 19.2 (4.4-42.6) to 14.3 (2.8-19.7)%. No significant falls in any of these parameters were noted following ulcer healing with ranitidine. Duodenal ulcer healing with sucralfate results in decreased acid secretory responses to vagal and pentagastrin stimulation.