We present an analysis of the vertical wavenumber and frequency spectra of atmospheric motions in the lower stratosphere observed using the Poker Flat, Alaska MST radar during a 5-day period in February 1986. Our purposes in this study are to identify the dominant contributions to the motion spectrum, to examine the hypothesis that the vertical wavenumber spectrum is controlled by saturation processes, and to determine whether the assumption of selxability of the wavenumber and frequency dependence made in several modelsof the atmospheric gravity wave spectrum is justified. Our results suggest that the majority of the energy at gravity wave periods is associated with inertia-gravity wave motions having an upward direction of propagation and dominant vertical wavelengths near 2 kin. Vertical wavenumber spectra indicate that there is excellent agreement with the saturation hypothesis, suggesting that saturation processes do indeed act to control spectral amplitudes at large wavenumbers. Two-dimensional results provide confirmation of separability of thewavenumber and frequency dependence of the gravity wave spectrum at sufficiently low frequencies. At high frequencies, however, our results suggest a departure from separability.