In this article we describe NVIZ, an interactive environment for system level investigation of neural function. NVIZ combines simulation, visualization and analysis in a single software system that encompasses five biological levels of organization, from ion channels to generated behavior. Once a simulation has been completed, cell-level output data can be visualized as height fields (presenting data from an entire population in a compact representation), or within an anatomical model of cat spinal cord. NVIZ provides a set of numerical analysis tools such as histograms, and a variety of 2D plots that permit both population and cell level analysis within the software system. Movement of a visualized limb segment is generated from activity of motoneurons, using a novel new algorithm called Net Neural Drive. The linked visualizations in NVIZ provide a powerful means to comprehend neuronal activity generated in complex models involving thousands of cells. The ability to design or modify neural circuits involving multiple populations, followed by simulation, visualization and analysis promotes rapid experimentation and the ability to digest massive amounts of time-varying simulation data. NVIZ is highly scalable, in terms of the number of populations, neurons, and limb segments. Using interactive tools, the visualization can be easily customized to focus on neural activity of interest. We demonstrate the application of NVIZ to understanding locomotion of a single limb joint, using a central pattern generator model.