The experiment was conducted in growing ram lambs to validate energy estimates of mixed diets obtained from in vitro incubation for gas production technique (RIVIGPT) through in vivo digestibility trials. Twelve Bannur ram lambs were divided in to four groups of three each. Four mixed diets comprised of rhodes hay (RH) and concentrate supplements (CS) in the ratio of 50:50 were tested in four periods in a switch over design. Each period lasted for four weeks with an adjustment period of three weeks and digestion trial(DT) for one week. The metabolisable energy (ME, MJ kg-1 DM) content of mixed diets at maintenance level of intake obtained by DT for diets-1,2,3 and 4 were 9.3, 8.8, 8.9 and 8.1 respectively. The corresponding ME values obtained from RIVIGP were 9.8, 9.0, 9.3 and 8.7 respectively. The mean ME output (Maintenance + weight gain) for the diets-1,2,3 and 4 were 4.4, 4.2, 4.3 and 4.2 respectively. The ME intake estimated from ME obtained from DT and RIVIGP for the diets-1,2,3 and 4 were 4.5 and 5.0, 4.2 and 4.7, 4.3 and 4.9, 3.9 and 4.6 respectively. Hence, the observed performance of growing ram lambs expressed in terms of the total ME output was accountable to the extent of 98 and 90% of ME intake estimates obtained from ME values derived by DT and RIVIGP respectively.