We investigated the effect of supplier and process uncertainty in a real time MRP environment. The environment modelled is supported by an ‘on line’ data monitoring and data collection system such that the activities of the shop floor and the receiving area are incorporated within the MRP database as they occur. The purpose of the model is to evaluate the operation of an integrated shop floor control/M R P system in a multi-level product structure environment which operates on a rolling horizon basis. The interaction of supply/process uncertainty with product structure and choice of lot sizing rule is examined for various cost factors. The presence of uncertainty is found to greatly complicate the operation of the MRP system. In addition, supply/process uncertainty combines with product structure and the choice of lot sizing rule, to impact the cost performance of the system. The significance of these interactions was confirmed by the formulation and testing of several hypotheses. Based on these results, some practical interpretations are also presented.