The wheat model Sirius was used to investigate the effect of variation in the capacity of one of the 2 vegetative pools of translocatable N on the grain protein content at harvest. The extension of Sirius developed by Martre et al [See Plant Physiology (2003) 133, 1959-1967] was also tested. The winter wheat cultivars tested in France were Arche, Recital, Renan and Tamaro; while those tested in New Zealand were Claire, Savannah, regency and Centaur. A set of simulations were also made using weather data from Lincoln, New Zealand, and the cultivars Belfield and Claire. The model gives a simple mechanistic framework that explains environmental and genotypic effects on grain protein content and composition. It is suggested that the variations in protein composition for winter wheat are due to different quantities of N per grain.