The MELISSA (Microbial Ecological Life Support System Alternative) project, conceived as a microorganism based ecosystem, is an early simplified model of a future biological life support system for manned space missions. The driving element is the recovery of edible biomass from waste, CO2 and minerals with direct use of light as a source of energy for photosynthesis. MELISSA is composed of four axenic compartments colonised by microorganisms and of a fifth compartment that is the crew on board the craft.This paper reports on the solution of mass balances over the entire MELISSA loop. The compartments within MELISSA are first analysed separately, each compartment being described by one or more stoichiometric equations derived from knowledge of the cells metabolic pathways. For each stoichiometric equation a key substrate entering a compartment is assumed to be completely exhausted at the outlet; process kinetics such as rates of biological reactions and mass transfer are ignored. Fifteen major components, five of which can be in the gaseous state, are considered and their chemical composition is defined for the four elements C, H, O and N. Secondly, after an analysis of the degrees of freedom of the system, the MELISSA loop is simulated at steady-state using a process simulator, each unit operation or unit process being modeled by a unit module.The results - mass fluxes and concentrations for each component in every stream of the system - are based on a three man crew. They show that due to the constraints imposed by diet it is not yet possible to obtain complete recycling of nitrogen and simultaneous regeneration of the atmosphere.