Abstract: Two in vivo digestion trials were conducted, by using a latin square 4x4 experimental design with castrated rams, to evaluate the effects of diet�s CP level, N degradability and Non-Forage Fiber Source (NFFS) on nutrient digestibility, N balance and energy value of sheep rations. In each trial, rams were fed at maintenance level four isocaloric-isonitrogenous and isofibrous rations, differing in main protein and/or NFFS source. At the first trial mean CP/ME ratio of the diets was 17 g MJ^-1 ME and at the second trial 13 g MJ^-1 ME. At both trials, the 1st ration contained Cotton Seed Cake (CSC) and Wheat Bran (WB), the 2nd CSC and Corn Gluten Feed (CGF), the 3rd Corn Gluten Meal (CGM) and WB and the 4th CGM and CGF. Data of both trials were analyzed in common as 2x2x2 factorial experimental design. Low N degradability (CGM) had positive effect on DM, OM, CP, NDF and ADF digestibility of the ration, whilst high N degradability (CSC) resulted in higher EE digestibility. Main protein source (CSC versus CGM) did not affect energy value and N balance of the diets. Those results suggest that an increase in Rumen Undegradable Protein (RUP) content does not negatively affect digestibility nor nutritive value of sheep rations. CGF significantly elevated CF digestibility, in comparison with WB. On the other hand WB increased EE and ADF digestibility of the rations. NFFS did not affect N balance nor energy metabolizability (q) of the diets. Rations having high CP/ME ratio had higher digestibility of CP in comparison with those having low CP/ME ratio; the opposite was true for EE, CF, NDF and ADF digestibilities. CP levelxN degradability interaction negatively affected energy value of the rations that had high CP level and high N degradability. Former suggest that when CP content is high then N degradability should be low otherwise ME of the ration is negatively affected. CP digestibility and coefficient q of rations containing WB and having high N degradability (N degradabilityxlowest suggesting that the combination of CSC and WB affect negatively CP digestibility and energy value of the ration. This could be explained, probably, by a reduced microbial CP synthesis, due to inadequate fermentable metabolizable energy of these rations, or lower RUP digestibility or both.