The spectral parameterization of gravity wave momentum deposition proposed by Medvedev and Klaassen has been incorporated into the Canadian Middle Atmosphere Model. Numerical experiments show that an anisotropic gravity wave source significantly improves the middle atmosphere circulation compared to an isotropic source. These results suggest that, at least in midlatitudes, the gravity waves primarily responsible for the momentum deposition at mesospheric heights are launched with observed phase velocities which are primarily in the same direction as the mean wind at tropospheric (source) heights. ¿ 1998 American Geophysical Union