The aim of the study was to examine the effects of the substitution of lucerne hay by untreated, urea-enriched and urea-ammoniated wheat straw on (i) dry matter intake (DMl), digestibility and nitrogen balance, (ii) in situ degradability, (iii) rate of passage from the rumen and (iv)  ruminal pH, volatile fatty acid and ruminal ammonia concentrations of sheep. Wheat straw was left untreated (WS), enriched with 1.5%  urea (UWS), or ammoniated with 5.5% urea in a stack for 8 weeks (AWS). It was then hammer illed (18-mm screen) and used to substitute lucerne hay (LH) at LH :WS / UWS / AWS ratios of 100: 0, 75: 25, 50: 50 and 25: 75 in a 3 X 4 factorial experiment involving the three straw treatments and four substitution levels. These diets were fed to 24 adult SA Mutton Merino wethers in a voluntary dry matter (DM) intake, in vivo digestibility and nitrogen (N) balance trial. These procedures were replicated twice to get four replicates per treatment. Rate of passage from the rumen (using SI Cr as external marker) and rumen parameters (pH, ammonia-nitrogen, volatile fatty acids), were investigated similarly. In situ organic matter (OM) degradability was determined according to a 3 X 3 factorial experiment, excluding LH from the analysis. Voluntary DMI declined linearly (P ≤ 0.01) by 224 (SEb = 33) g/sheep/d for each increment increase of WS/UWS/AWS. A quadratic tendency (P = 0.09) was also observed, indicative of some associative action between LH and the straws. This trend was consistent for all straw treatments. No significant effect of LH level on apparent DM and OM digestibility was found, while apparent NDF, ADF, hemicellulose and cellulose digestibility increased linearly (P ≤ 0.01) with decreasing amounts of LH. Apparent N retention significantly (P ≤0.01) decreased with decreasing amounts of LH in the diet. This trend could be attributed directly to higher N intake levels on the diets high in LH, as apparent N retention as percentage of N intake was independent of LH level. The effective OM degradability of the diet decreased linearly (P ≤0.01) with a decline in level of LH with the substitution of LH by WS and UWS. When LH was substituted by AWS, there was also evidence of a quadratic response (P  ≤0.01), suggesting some associative action. Level of LH had no significant influence on outflow rate from the rumen, although it tended (P ≤ 0.12) to decrease with increasing amounts of straw. Ruminal pH increased (P ≤0.05) on diets including higher amounts of straw, while ruminal volatile fatty acid concentrations followed an opposite trend (P ≤ 0.01). Ruminal ammonia N decreased (P ≤ 0.01) with the substitution of LH by WS and AWS but it was curvilinearly influenced by the substitution of LH by UWS. In general, ammoniation improved (P 0≤.10) voluntary DM intake, as well as the apparent digestibility coefficients of DM, OM and the various fibre fractions relative to untreated WS. Rate of passage from the rumen was significantly (P 0≤.05) shorter on AWS than on untreated WS. This study accorded with previous results that ammoniation, in general, improves voluntary DMI, apparent digestibility of fibre fractions and apparent N retention. No conclusive associative effects between LH and WS, UWS or AWS were observed with regard to these factors or rumen parameters, apart from the results regarding voluntary DMI, and the in situ OM degradability in the case of AWS.