Sixteen sheep were continuously fed diets based on straw, straw and barley grain, or NaOH-treated straw, each with adequate minerals and with or without additional urea (7·3 g N/day for the straw and straw and barley diets; 14·4 g N/day for the diet containing NaOH-treated straw) infused intra-ruminally. [14C] tracers were used to measure the rate of irreversible loss (i.e. synthesis) of plasma urea and its transfer to the rumen and to the remainder of the gastrointestinal tract in 4–6 sheep per treatment. [15N] tracers were also used to make these measurements in four sheep fed the straw, straw and barley and straw and barley with intra-ruminal urea infusions. Inclusion of the barley grain in the diet increased digestible organic matter intake by c. 70%. Replacing the straw with a greater amount of NaOH-treated straw increased digestible organic matter intake by 43% in the absence of additional urea, and by 65% when urea was infused intra-ruminally. Rumen ammonia concentrations were 37 and 31 mg N/l when straw or straw and barley diets respectively were provided and were reduced to 3 mg N/l when treated straw was given to the sheep. These concentrations exceeded 223 mg N/l when urea was infused intra-ruminally. Rate of irreversible loss of plasma urea measured using [14C]urea ranged from 3·7 g N/day in sheep given the treated straw diet to 16·8 g N/day when urea was infused intra-ruminally, while this rate was correlated with plasma urea concentration. In the three diets where it was measured, rate of irreversible loss of [15N]urea was on average 0·73 of the rate of irreversible loss of [14C]urea. Measurements made with 14C and 15N tracers of plasma urea transfer to the rumen were similar. This transfer measured with 14C tracers ranged from 1·4 to 2·4 g N/day for the three diets without additional urea, while providing urea by intra-ruminal infusions reduced the transfer to 1·1–1·4 g N/day. Transfer of plasma urea to the rumen was not changed by the higher digestible organic matter intake associated with either inclusion of barley grain in the diet or provision of a greater amount of more highly digestible NaOH-treated straw. These studies indicate that plasma urea transfer to the rumen is not always increased by greater organic matter fermentation in the rumen.