Summary: The long-term behaviour of the aortic-valve allograft seems to be dependent on the maintenance of its matrix. Protein and collagen synthesis was studied in rat aortic-valves. Quantitative and qualitative methods proved the production of at least two protein pools. One protein pool is localised intracellularily with a 'turn-over' time of about 2 weeks, and a second pool is localised extracellularily with a 'turn-over' time of at least 8 weeks. The latter protein pool mainly consists of collagen.