Ingestion of soft faeces derived from caecal contents, caecotrophy, in herbivorous small mammals is considered an adaptation to the metabolic disadvantage of small body size, especially when feeding on diets of low quality. We investigated daily activity patterns in captive Brandt's voles (Lasiopodomys brandtii), including feeding, locomotion, caecotrophy, and defaecation, by continuous 24 h visual observation; and estimated the contribution of soft faeces ingestion (caecotrophy) to intake of protein and energy. Brandt's voles ingested 68.8 +/- 7.4 fecal pellets per day, averaging 17 +/- 2% of total faeces produced. The amount of faeces ingested did not differ between female and male voles or between night and day time. All animals showed average 3 h ultradian cycles in behaviour during the course of the day and night. The contributions of caecotrophy to the dietary intake of crude protein and metabolizable energy were estimated respectively as 9 and 8% on a high-protein, easily digested commercial rabbit pellet diet. However, the importance of caecotrophy to the field voles is likely to be higher on a natural diet of lower nutrient density. The rhythm of caecotrophy in voles depended mainly on the rhythm of the colonic separation mechanism in the proximal colon and passage in the distal colon, and may be regulated by feeding and other activity rhythms. Ultradian rhythms in caecotrophy helped to minimise potential conflicts in utilizing the gut, especially in balancing the caecal fermentation and salvaging nutrients contained in caecal bacteria.