This study investigated the effects of replacing rapid-release nitrogen (N) from urea with a graded level of slow-release N (Optigen® II) source on intake, digestibility, rumen fermentation and microbial protein synthesis, when sheep were fed a poor-quality roughage diet. Five rumen cannulated wethers were used in a 5 x 5 Latin square experimental design. The treatments had various proportions of urea to Optigen® II (0 : 100, 25 : 75, 50 : 50, 75 : 25 and 100 : 0), with the same inclusion level of starch and a mineral premix on an iso-nitrogen basis. The 25% urea : 75% Optigen® II treatment showed significantly higher intakes of dry matter, organic matter, neutral detergent fibre and digestible organic matter than in the other treatments. No differences were recorded for apparent dry matter digestibility, organic matter digestibility and neutral detergent fibre digestibility among the treatments. However, sheep on the 100% Optigen® II treatment had a significantly lower apparent nitrogen digestibility. No differences were observed for ruminal pH and volatile fatty acid concentrations among the treatments, except for butyrate and isobutyrate concentrations. The butyrate concentration of the 100% Optigen® II treatment was significantly lower than the other treatments, whereas the isobutyrate concentration was significantly lower than in Treatments 2 (75% urea : 25% Optigen® II) and 5 (100% Optigen® II). The rumen NH3-N concentration of the 100% Optigen® II treatment was significantly lower than the 100% urea treatment at two and four hours after infusion. Based on biological responses, results suggest that up to 75% of urea could be replaced with Optigen® II in supplements.