Four sheep were maintained by infusion of volatile fatty acid (VFA), mineral and buffer solutions to the rumen and casein to the abomasum. After a 3 week period in which control measurements were made, glucose was introduced gradually to the rumen infusion mixture until glucose replaced about 65% of VFA energy. Further measurements were made in the fourth week of glucose additions after which the control VFA infusions were re-established and measurements repeated. Urea kinetics were measured with [14C]urea. Rumen ATP concentrations were used as an index of microbial growth and increased from 1.5 nmol ml-1 on control treatments to 46 nmol ml-1 when glucose was given. The presence of a rumen fermentation resulted in a 40% decrease in plasma urea concentration but was without effect on rumen ammonia concentration. Urea irreversible loss rate decreased from 18.0 to 12.8 g urea day-1 when glucose was given. This, however, was matched by a decrease in urinary urea excretion, with the result that urea degradation, obtained by difference, remained constant throughout the three treatment periods. Daily nitrogen (N) retention increased from 1.0 to 2.7 g when a rumen microbial population was present. A model of urea and NH3 transactions for the two dietary situations is presented. Calculations indicated that 0.40 of the casein N supply was reutilized as urea by the rumen micro-organisms. It is suggested, however, that the improvement in N retention resulted from a change in protein:energy ratio of the infused nutrients rather than from an enhanced supply of protein to the intestine. The use of endogenous urea clearance as an index of epithelial permeability to urea is questioned.