Eighteen Holstein bull calves averaging 100 kg were individually housed and radomly assigned to nine dietary treatments. Isonitrogenous diets, containing approximately 12% protein (DM basis), were offered free choice. Treatment 1 consisted of ground ear corn and liquid protein supplement (LPS-50) with 50% protein equivalent from urea. In the eight remaining treatments, corn ears were gradually replaced by processed poplar wood pellets (Bender-Heaney process) so that Treatment 9 consisted of processed wood and LPS-50 only. Increments of processed wood dry matter were approximately 10%/treatment. Weight gains and feed consumptions were recorded at 14-day intervals over the 98-day trial. Blood samples were taken for 3 consecutive days prior to slaughter. Digestive organs were observed for abnormalities and reticuloruminal contents were analyzed for pH, protein and volatile fatty acids. Overall daily gains (kg) and feed conversion ratios for Treatments 1–9 were 1.14, 5.97; 1.26, 5.81; 1.35, 6.01; 1.16, 6.73; 1.01, 7.31; 0.91, 8.05; 0.52, 11.44; 0.16, 27.23, respectively. There were no treatment effects on acetic, propionic and butyric acid concentrations in the reticulorumen. However, as the proportion of processed wood increased, there was a significant decrease (P < 0.05) in the concentrations of valeric and isovaleric acids. The curve for blood glucose was quadratic (P < 0.05), reaching maximum at the 20% level of processed wood and decreasing gradually thereafter. The rise of blood urea nitrogen was linear with increased values for treatments with higher proportion of processed wood in the ration. No consistent diet-related histopathological effects on the digestive organs were observed. Key words: Processed wood, NPN, iso-acids, BUN, calves