Supplementation of untreated, urea-supplemented or urea-ammoniated wheat-straw diets with sources of readily fermentable energy (maize-meal; 0 and 20% of the total diet) and rumen-undegradable protein (fish-meal; 0 and 5% of the total diet) was investigated in a 3x2x2 factorial experiment, involving an intake and in vivo digestibility trial with 48 adult S.A. Mutton Merino wethers. Straw dry matter (DM) intake on ammoniated wheat-straw diets was 27 and 22% higher (P ≤0,01) than on untreated and urea-supplemented diets, respectively. No significant difference was found between the latter treatments. The inclusion of 5% fish-meal stimulated (P ≤0,01) straw DM intake by 13% relative to the control diets without fish meal. Incorporation of the 20% maize-meal supplement alone, or in combination with 5% fish-meal, did not result in any further improvement in straw DM intake. The apparent digestibility of organic matter (OM), cell-wall constituents (CWC), acid detergent fibre (ADF) and hemicellulose was higher (P ≤0,01) on the urea-ammoniated wheat-straw diets than on either the untreated wheat-straw diets or the urea-supplemented wheat-straw diets. No differences were obtained between the latter treatments. The apparent OM digestibility of the wheat-straw diets was improved by the inclusion of 5% fish-meal and/or 20% maize-meal. Apparent digestibility of CWC and ADF was not particularly affected by the inclusion of fish-meal or maize-meal. Combined supplementation of the straw diets with both 5% fish-meal and 20% maize-meal tended to suppress the apparent digestibility of CWC and ADF. Apparent hemicellulose digestibility of the wheat-straw diets was increased (P ≤0,01) by 4,5% due to the inclusion of maize-meal. Apparent nitrogen (N) digestibility and N-balance was involved in significant (P ≤0,01) three-factor interactions. Urea supplementation improved (P ≤0,01) N-digestibility compared to untreated wheat-straw diets. Urea ammoniation similarly improved N-digestibility in relation to untreated wheat-straw with or without supplementary maize-meal. The apparent N-digestibility of urea-supplemented wheat-straw tended to be higher than on urea-ammoniated wheat-straw, but the only significant (P ≤0,01) difference occurred between the diets without maize-meal and/or fish-meal. Nitrogen balance was generally improved in sheep receiving diets containing urea-ammoniated wheat-straw, and fish-meal alone or combined with maize-meal. No significant differences in N-balance were found between the untreated wheat-straw diets and corresponding urea supplemented wheat-straw diets.