Basal metabolic rate (BMR) was determined in 56 crossbred sheep, up to 10 observations being made on each animal between 1 week of age and 2¼ years. The level of feeding was varied amongst the sheep so that there was a wide range in growth rate at each age. BMR was estimated as heat production under standard conditions of fasting. Trends during fasting were studied in four sheep at ages 3 weeks, 2 months and 9 months. The effects on BMR of body weight (or fat-free weight), age, prior growth rate and prior nutrition were examined statistically by estimating the parameters of a series of model equations by a least squares iterative method. Analysis of lamb and sheep data separately and combined showed that all these variates contributed significantly to BMR. Of the variance of BMR, 89% was accounted for in a body weight term, kgx, in which the value of x was not significantly different from ¾ if one or more of the other variates were in the model; x was unity when fat-free weight was used instead of body weight. If body weight was used alone, x was smaller for both lambs and weaners, being c. 0.60; with fat-free weight the values for lambs and weaners were 0.71 and 0.96 respectively. Age, growth rate and level of feeding were of approximately equal importance, together accounting for a further 6% of the total variance. BMR declined by c. 8% per annum and was affected to the extent of 2.8 kJ per gram body weight gain and 46 kJ per MJ digestible energy intake before fasting (all values per 24 hr). Thus an increase in growth rate in a lamb from zero to maximal (0.3 kg/day) caused BMR to increase by 50%, and an increase of food intake by 1 kg/day in an average adult sheep caused BMR to increase by 10%. For any given set of these variates, BMR was 23% higher in milk-fed lambs than in weaned sheep. An equation was derived for sheep in general; the residual standard deviation was c. 300 kJ/24 hr, or 7-8% of BMR in an average adult sheep. Some evidence was cited to show that this equation may be used to predict BMR in growing and adult cattle by multiplying the whole expression by 1.3.