An analysis of compact natural circulation steam generators for nuclear power plants has been made to facilitate the prediction of certain dynamic characteristics of the systems in terms of their pertinent geometric parameters. A simple two-phase annular flow model is proposed to permit analytical solutions for the thermal driving head and friction pressure loss in the riser. The results of this model have been interpreted in terms of the Martinelli correlation and a favorable comparison is presented. The equations from the analysis have been programmed for solution by the IBM 650 digital computer and several steam generator configurations studied with regard to geometric parameters and also thermodynamic operating state. The main consequence of rapid power changes with these specific units is the carryover of moisture to the turbines which implies adequate separator margin can not be provided for. The capability of the evaporator to supply adequate steam for any desired power change has not been questioned; however, it is noted that a reduction in downcomer flow during a transient could reduce its capacity to do so.