The 1960s movement of Metabolism in architecture utilized, albeit critically, biological concepts and models for the design of built environments. The architects of the group recognized architectural products as incomplete entities and with continuously changing elements similar to an evolving population of organisms. Their imagination led to the generation of new spatial systems and early forms of computational methodologies for architectural and urban design. As a figure closely related to the movement, Kenzo Tange's model of vitality is a case in point and is at the heart of a visually coded urban design system he developed for Skopje. Vitality is organisms' continuance of life and has both changeable and static components. For Tange, it was the key characteristic of continuous flows of urban movement as well as the change and growth in how architectural and urban systems respond to the dynamics of population, production and communication. In many of his grand projects, Tange modeled the linear development of urban flows. For Skopje, he formulated a code that combined a simple shape vocabulary of urban space and rules for movement intersections. With reference to Tange's personal notebooks, sketches and lecture notes from the archives, this paper aims to delineate the systematic implications of biological models in the proposal Tange's team prepared for Skopje. His discourse for a systems approach positions Tange in the history of computational architectural and urban design.