Small bowel transit time was determined in surgical patients by measuring the rise in breath hydrogen excretion after the ingestion of lactulose. 50 patients were studied on the day before surgery, the study being repeated on the 1st postoperative day. The concentration of hydrogen in expired air was measured before and at 15-min intervals after the ingestion of 20 g lactulose. The mean ( ± SD) pre-operative mouth-caecum transit time was 60.9 ± 30.2 min. The effect of intra- and extraperitoneal operations on the postoperative transit time was evaluated in four groups of procedures: (a) non-abdominal (n = 9); (b) abdominal wall (n = 11); (c) intraperitoneal extra-intestinal (n = 11), and (d) intraperitoneal intra-intestinal (n = 9). Only patients who underwent intraperitoneal operations (groups c and d) experienced a significant delay in their mouth-caecum transit time on the 1st postoperative day (p = 0.008 and 0.004). The transit time in patients who had extraperitoneal procedures (groups a and b) did not show a significant variation. The H2 breath test proves to be a useful tool in the clinical study of intestinal motility.