The objective of this study is to determine the effects of Leucaena leucocephala (Leucaena) and Gliricidia sepium (Gliricidia) supplementation on the outflow rate and microbial protein yield of sheep fed with ammoniated rice straw (ARS) diet. Twenty-four growing native sheep with mean body weight of 13.48 kg were equally distributed to three treatment groups following a Completely Randomized Design (CRD). They were fed ad libitum with ARS alone or supplemented with either Leucaena or Gliricidia that accounted for 20% of the daily dry matter (OM) intake. ARS consumption remains the same among treatments, but the addition of Leucaena and Gliricidia increased total OM consumption which eventually resulted to higher (P<0.01) growth performance and more (P<0.05) efficient feed utilization. Supplementation also resulted to faster (P< 0.05) flow rate of rumen digesta, 5.1 and 4.8%/hr, for the Leucaena and Gliricidia, respectively, compared to 3.8 %/hr in the ARS group. The significant improvement in N utilization and more digestible organic matter intake (DOMI) brought about by the inclusion of Leucaena and Gliricidia to ARS resulted in increased (P<0.05) microbial N yield. Efficiency of microbial N supply (EMNS) between Leucaena and Gliricidia groups were not significantly different, but both were significantly higher than the 29.41g MN/kg ROOM for ARS. After 90 days feeding, heavier (P<0.05) lambs and higher ADG were observed in the forage supplemented groups. The significant improvement in the live weight of lambs also resulted in a more efficient feed utilization. Animals fed with Leucaena and Gliricidia required 13 g DM to produce 1-g gain per day, which is about 10 units better (P<0.05) than the 22 g in the unsupplemented group. Result suggests that faster outflow rate of rumen digesta increased the influx of DOM and microbial protein supply in the small intestine that could have been the major contributory factor to the higher consumption and growth performance in the forage supplemented groups.