1. Studies were made of the extent to which p-cresol, catechol, quinol and orcinol infused through rumen or abomasal cannulas to sheep were recovered in their urine. 2. Rumen fermentation of dietary phenolic compounds caused the excretion of simple phenols in the urine. In decreasing order of magnitude these were: p-cresol, catechol, phenol and 4-methylcatechol with only traces of quinol and orcinol. 3. The percentages of rumen-infused p-cresol or orcinol recovered as increments in the urinary phenol outputs of sheep (94 and 99% respectively) following infusion showed that rumen degradation of these phenols was negligible. 4. After rumen infusion of catechol and quinol, mean recoveries of these phenols in urine were only 55 and 77% respectively. Possible reasons for these incomplete recoveries are discussed. 5. Studies were also made of the use of the urinary phenol output of phenols characteristics of particular forages as indices of their voluntary intake by sheep. Calluna vulgaris L. (Hull) (heather) may contain 1300-3600 mg/kg dry matter (DM) of orcinol and 200-800 mg/kg DM of quinol as beta-glycosides. When heather was offered ad lib. to sheep given one of five levels of grass, linear relationships were found between heather intake and urinary quinol and orcinol outputs. 6. The urinary output of aromatic acids was also determined when sheep ate grass and heather. Urinary phenylacetic acid output was linearly related to grass but not to heather intake. The relationship between urinary phenylacetic acid output and grass intake could vary with different forages but that between orcinol output and heather intake was considered a useful index of heather intake. 7. Methods for the assay of urine phenols are discussed.