Energy transfer rate constants were measured for excited rovibrational levels of Na2 (X1Σ+g). Stimulated emission pumping was used to excited the levels ν=33～51, J=11 via A-X transition. Laser induced fluorescence was used to follow the collision dynamics. Energy transfer processes induced by collisions with Ar and H2 were investigated. The decay curves for the parent level populations gave good fits to single exponential function. At ν=33～51, the total transfer rate constants increase linearly with vibrational quantum number. Parameterized expressions for the (48, 11)- to -(47, J) rate constants were fitted to the fractional populations of the satellite lines. This produced sets of relative rate constants. Absolute rate constants were then obtained by normalizing the sums of the relative rate constants to the total removal rate constants. For Na2(ν)+Ar, no multiquantum vibrational transfer was detected. For Na2(ν)+H2, a significant fraction of the initial population of highly vibrationally excited Na2(X ν=48) relaxes to lower vibrational level (Δν=-5). The time scale is much shorter than the known collisional lifetimes of the intervening vibrational levels and thus a sequential single-quantum relaxation mechanism can be explicitly ruled out. For ν=48, at least 40% of the initially prepared population, undergoes multiquantum vibrational relaxation. We discuss possible explanations of this result.