I . Energy exchanges and other physiological functions associated with eating and rumination were determined in four experiments. Sheep were given chopped, dried grass (DGC), pelleted, dried grass (DGP) or fresh grass (FGC).2. In Expt I a preliminary study was made using all three diets. The dry matter (DM) of DGP was eaten significantly faster than that of chopped diets. Sheep salivated most during eating and ruminated longest when given DGC. Rates of contraction for the reticulo-rumen did not differ significantly between diets during idling and rumination, but were significantly faster during eating with DGP. The apparent energy costs of eating were 17, 109 and 176 kJ/kg DM eaten for DGP, DGC and FGC respectively, but these probably underestimated the true energy cost.3. Expt 2 compared DGP and DGC at two levels of intake. The mean energy costs of eating DGP and DGC were 23.5 and 267 kJ/kg DM respectively. There was no consistent relationship between the energy cost of eating and the duration of the meal. The proportion of time the sheep spent ruminating DGC was about 23 yo but less than I yo for DGP. There was no significant relationship between heat production and the time spent ruminating.4. In Expt 3 four sheep were offered fresh grass and, later, an equivalent DM intake after the material had been dried. The sheep ate the dried meal significantly faster. The mean energy costs of eating were 208 and 346 kJ/kg DM for DGC and FGC respectively. In this experiment the sheep ruminated significantly longer when given FGC, and the energy cost of rumination was 0.11 kJ/min. 5.Increases in heat production during and after fistula-feeding were only 2-8 % of those obtained during eating, indicating that nearly all the increase in heat production during eating could be attributed to the energy cost of eating per se.6. The contribution of the energy costs of eating and rumination to the heat increment of feeding and the energy requirement for maintenance of sheep are discussed.The heat production, and thus the energy requirement for maintenance for sheep is usually considered to be the sum of fasting metabolism, the heat increment of feeding, the energy cost of any activity additional to that made during fasting, and the energy cost of maintaining homeothermy. In comfort and confinement, as in a calorimeter, the last two of these factors are taken as zero, the heat increment of feeding is considered to be a function only of the quantity and quality of the food ingested, and the energy expenditure associated with the activity of eating is assumed to be negligible (Agricultural Research Council, 1965 , 1972). It is unlikely that these differences can be attributed entirely to the energy cost of standing and walking at pasture (Clapperton, 1964) and it has been suggested that they could, in part, be due to the increased energy cost of grazing uncut tine, Trinidad, West Indies.

1. A detailed analysis was made of the rumination behaviour of one caged sheep on four combinations of diet, ranging from 100% hay to 100% concentrate meal.2. When hay was fed there was no relationship between the quantity of it ingested and the amount of rumination, which averaged 8 hr. daily. When concentrates were fed alone rumination time fell to 2½ hr. per day and much of this was ‘pseudorumination’.3. Evidence is produced to show that the number of chews and not the time chewing is the accurate way of measuring rumination quantitatively. This particular sheep made an average of 39,000 rumination chews per day when hay was fed.4. The duration of the intervals between boli was constant at 15% of total rumination time, but the rate of chewing varied from 83–99 chews per minute, being slower during the midnight to noon half of the 24 hr., when most rumination occurred.5. Rumination periods last from under 1 min. up to about 2 hr.6. Tiring during a rumination period was reflected by an increase in the intervals between boli but the number of chews per bolus remained unchanged.7. The first bolus of each period of rumination was chewed less than those following. This sheep averaged fifteen periods of rumination per day, a daily total of 500 boli were regurgitated. There were 2500 chews per period and seventy-eight chews per bolus.

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