Transient-analysis plasma-physics/electromagnetic codes were used to model microwave tube circuits excited by the sudden or rapid buildup of an unmodulated electron beam. The objective was to assess the value of Mission Research Corporation's codes "SOS" and "MAGIC" (for three- and two-dimensional analysis, respectively) for analysis of oscillation in microwave tubes. For the 2-D case the resonance frequency and the beam-loadingQof a cavity were obtained and corroborated by a steady-state electromagnetic code. For the 3-D cases the computed resonance frequencies in a resonated section of coupled-cavity TWT were corroborated by cold-test measurement. Despite the coarse frequency resolution inherent in the relatively short-duration transient analysis perfomed for a higher order antisymmetric mode, it was possible to isolate several of the resonances sufficiently to determine the negativeQdue to beam loading. Thus the capability of these programs to perform quantitative stability analysis on microwave tube circuits with beam was demonstrated. To be of practical use in routine tube design and analysis, however, a more efficient and user-friendly code must be developed.