The problem of shrinkage in injection moulding has been studied by means of a new technique based on strain gages placed on mould surface before injection. The local shrinkage development from the moment it starts inside the mould to soon after ejection can be followed by means of this method. With reference to a simple rectangular cavity, the effects of different holding pressures, of geometrical constraints placed inside the mould, of position in the cavity and of total cavity length on local shrinkage of injected PS samples are analysed. Shrinkage inside the mould is, indeed, registered: it starts later if higher holding pressures are applied; furthermore, any factor which produces and enhancement of shrinkage evolution before complete solidification gives rise to a corresponding increase of final shrinkage.