1. Large intestine electromyograms were recorded from four pigs receiving successively three diets containing different amounts of fibre (standard cereal diet, milk substitute and milk plus 170 g bran/kg). For each diet the mean retention time of a marker added to one meal was determined. Three occurrences of spontaneous constipation were also studied.
2. Slow waves were observed in the colon only during 10% of the recording time at a frequency of 10.3 ± 1.1/min. Two kinds of spiking activity were recorded on the colon: short spike bursts of 0.5–2 s duration and long spike bursts of approximately 7 s duration. Long spike bursts alone were recorded on the caecum.
3. The mean retention time of the marker was 40 h on the standard diet, 120 h on the milk-substitute diet, and 66 h on the milk plus bran diet.
4. On the standard diet, approximately 1500 colonic long spike bursts were recorded daily and short spike bursts occupied 15% of the recording time. On the milk diet, the number of long spike bursts decreased significantly (P < 0.01) to 500/d and the short spike bursts increased significantly (P < 0.05) to take up to 21% of the time. Intermediate values (1100/d and 19%) were observed when bran was added to the milk diet. The lowest values for long spike bursts (300/d) and highest values for short spike bursts (44% of the time) were observed in constipation.
5. It is concluded that long spike bursts are associated with propulsion of intestinal contents and that short spike bursts are non-propulsive.