The low frequency variations are closely related to thestationary waves and, in a sense, they may even be viewed as the timevarying part of the stationary waves themselves. Therefore, an understanding of the maintenance of the stationary waves is a prerequisite for a thorough understanding of the low frequency transients.The Eliassen-Palm (EP) cross-section and the three-dimensional waveactivity flux, which was derived by Plumb (1985) for the linear quasi-geostrophic stationary waves in a zonal flow, are used as the diagnostics for the output of a three-dimensional spectral model of the linearand nonlinear steady-state baroclinic primitive equations. The resultsshow that the EP cross-section and the wave activity flux are usefulfor diagnosing the vertical and horizontal propagation of the waves andthe significance of the nonlinearity. However, full attention should bepaid to the effect of the nonlinear interaction when diagnostic analysisis made of the observational data.