The uncertainty inherent in material requirements planning (MRP) systems is of two general types: demand uncertainty and supply uncertainty. Supply uncertainty is a result of late deliveries (timing) or order shrinkage (quantity) or both. The purpose of this paper is to examine the impact of operating policies on an MRP system given supply uncertainty resulting from timing factors, specifically the uncertainty caused by variability in the lead time of purchased parts. Experiments arc conducted to assess the impact of four factors: the amount of lead time variability, the amount of safety stock or safety lead time, the lot-size rule, and the holding cost and lateness penalty, on the total cost of the MRP system given supply/timing uncertainty. An MRP/Production simulator is employed to perform the experiments. Practical guidelines for the manager of an MRP system with supply/timing uncertainty are presented.