An example of the current version of BIOMOD, an interactive computer-graphics system for biological modeling. The example models water and solute distribution between the intravascular and extravascular spaces of the body. BIOMOD operates on a graphic console comprising a CRT screen, RAND Tablet, and keyboard. BIOMOD allows the user to draw block diagrams, handprint or type text, push displayed buttons, and drag labels, while providing immediate interpretation and validity reports. A user may represent a model by a block diagram, each component of which may be defined by another block diagram, by CSMP statements, or--when BIOMOD is completed--by chemical equations, differential equations, or data curves. Hierarchy facilitates devising complex models a portion at a time. All modeling languages used are internally translated to IBM's CSMP/360 simulation modeling language. During the simulation, the user may display curves for different variables, rescale, alter simulation parameters, and immediately rerun or continue the simulation.