Examines the process of perceiving and predicting the behavior of complex systems. BIOMOD, an interactive, graphical digital computer system with a remote terminal, has software designed specifically for use by biologists untrained in computer programming. During model construction, the user draws a flow chart and defines the model. During model simulation, he adjusts parameters and observes the output variable plots, comparing them with desired or laboratory results. The user can modify the model and go again to simulation. The key to the system's power is the facile, biologically oriented interface language that places the burden for language translation and problem organization on the software. An osmotic balance problem is given as an example. The actual iterative process could be done by batch processing in a month. Using BIOMOD the biologist can solve it in an afternoon and simultaneously gain a sound, intuitive understanding of the mechanisms, sharpening his perception of the basic problem.