Estimations of purine derivatives excretion and urinary-nitrogen loss were used to test the response of rumen fermentation to supplementation of straw with different sources of carbohydrate. Two groups of Rasa Aragonesa ewes (44 (s.e. 0·75) kg live weight were given ad libitum basal diets of either ammonia-treated (ATS) or urea-supplemented (USS) barley straw, with 12 animals per basal diet group. Three supplements, barley grain, sugar-beet pulp or grass hay, respectively, were given to each basal diet group, giving a total of six dietary treatments with four animals per treatment group. Four levels of supplementation were studied (150, 300, 450 and 600 g air dry matter per day), one in each of four experimental periods. Each 45-day experimental period comprised 38 days of adjustment followed by a 7-day measurement period. Digestible organic matter (DOM) intake was higher in animals receiving ATS than in animals receiving USS (504 v. 474 (s.e. 21·1) g/day, P < 0·005) and higher in animals receiving barley grain and sugar-beet pulp than in those receiving grass hay (512 and 496 v. 370 (s.e. 25·9) g/day, P < 0·005). DOM intake also increased with the level of supplementation and this increase was greater with barley grain (504 to 634 and 314 to 554 g/day for ATS and USS) and sugar-beet pulp (440 to 582 and 315 to 522 g/day) than with grass hay (430 to 407 and 267 to 370 for ATS and USS). Urinary excretions of hypoxanthine, xanthine and uric acid were not affected by the experimental treatment whereas allantoin excretion (y, mmol) increased in response to DOM intake (x, kg) (y = 13·72 × − 0·26; r = 0·79; P < 0·001; no. = 96). The response in allantoin excretion was mainly explained by the increase in DOM intake. However when data were expressed per unit of DOM intake significant differences were still evident. Allantoin/DOM intake (mmol·kg) ratio and calculated microbial nitrogen (g·kg DOM intake) supply were lower with USS diets and sugar-beet pulp supplemented diets (P< 0·05) and increased significantly with level of supplementation (P < 0·001).