Four female sheep nourished wholly by infusions of volatile fatty acids (VFA), buffer and minerals to the rumen and casein to the abomasum were given in addition infusions of supplementary energy calculated to increase the energy input to the rumen by 30%. The design was a Latin square and the supplements given were (a) nil, (b) a standard VFA mixture similar to the basal infusion, (c) butyric acid alone and (d) glucose. Measurements were made of nitrogen retention and rumen fermentation characteristics and the kinetics of urea metabolism were measured over 24 h by means of a single injection of [14C]urea. An active microbial fermentation was established in the rumen in response to the infusion of glucose and estimates of microbial protein synthesis derived from urinary purine excretion agreed well with those calculated from stoichiometric principles. The presence of a microbial population in the rumen resulted in a decrease in urinary urea excretion and reductions in plasma urea concentration, urea pool size and rumen ammonia (NH3) concentration. Infusion of the mixed VFA or butyric acid supplements had no effect on these indices of urea metabolism. Measurements of urea irreversible loss rate showed high variability and the mean values did not differ significantly between the four treatments. Urea degradation in the gastrointestinal tract was also highly variable but increased, on average, by 2.4 g day-1 on the high-energy treatments. Examination of regression relationships between these variables also indicated a difference between the glucose treatment and the others in the metabolic fate of the NH3 derived from urea hydrolysis. It is concluded that urea degradation increased in response to additions of energy but did not differ according to the nature of the supplements supplied. In the glucose-supplemented group, the NH3 arising from degraded urea was incorporated into microbial protein and so removed from the urea-NH3 cycle; when additions of mixed VFA or butyric acid were given, the NH3 arising in hydrolysis appeared simply to be reabsorbed as NH3 and to contribute anew to urea formation.