Phenolic monomers identified in common roughages were included in diets fed to rats both as mixtures, to mimic phenolic composition of roughages, and individually at various levels. Three performance and digestion/metabolism trials were conducted. Dry matter, protein, total carbohydrate, cellulose, and soluble carbohydrate (dextrose plus starch) digestibilities were measured. Urine and fecal samples were analyzed for phenolic monomers in trial 1. Inclusion of phenolic monomers (p-coumaric, ferulic, protocatechuic and salicylic acids, and vanillin) in rat diets resulted in reduced feed intake with a linear trend toward decreased intake with increasing dietary phenolic (p-coumaric and ferulic acids) concentrations. Average daily gain also exhibited a similar negative linear relationship. p-Coumaric and ferulic acids did not differ from one another in their effects, whereas salicylic acid had greater negative effects and vanillin was not different from the phenolic-free control. Some differences in digestibility of individual nutrients were observed, but were small in magnitude and did not correlate well with the altered growth responses. Analysis of urine and feces revealed low recoveries of phenolic monomers, suggesting extensive metabolism of phenolics by rats after intestinal absorption. Potential exists for phenolic monomers to be intake- and growth-depressing agents in both animal and human diets.