The fat component of meals has been thought to make a major contribution to the colonic response to feeding. We have combined gamma scintigraphy and radiotelemetry to noninvasively study the response of the normally inaccessible proximal colon after ingestion of either a high or low fat meal. Separate studies were performed to measure the rate of passage of the same meals through the whole gut. Gastric emptying and small bowel transit of the two meals to the colon was similar, 50% of meal marker reaching the ascending colon 4.8 +/- 0.2 and 4.5 +/- 0.3 hr after the high and low fat meals respectively (N = 8, difference not significant). The low fat meal caused a consistent increase in motility index, which rose from a basal value of 1.0 +/- 0.3 to 2.6 +/- 0.7 mm Hg in the 2 hr after the meal (N = 8, P < 0.01). Response to the high fat meal was less consistent, motility index increasing from 1.6 +/- 0.6 basally to 2.3 +/- 0.7 mm Hg postprandially (N = 8, P = 0.21). Despite these increases in motor activity there was no net caudal propulsion of colonic contents after either meal. The geometric center was comparable, being 3.2 +/- 0.4 and 3.7 +/- 0.4 before the high and low fat meals. This did not change significantly after either meal, being then 3.5 +/- 0.4 and 3.6 +/- 0.4 2 hr after the high and low fat meals, respectively. We conclude that in normal subjects equicaloric high and low fat meals transit the whole gut at a similar rate.(ABSTRACT TRUNCATED AT 250 WORDS)