We study the problem of inventory control and supply planning for assembly systems under lead time uncertainty. The aim is to minimize the average holding cost for the components while keeping a high customer service level. The customer demand is fixed, but the component lead times are random. The lead times of the different types of components follow the same distribution probability, and the holding costs of the ordered quantities are the same. A periodic "lot for lot" policy for supply planning is studied. The decision variables are the planned lead times of components. A generalized Newsboy model is proposed for optimization. This model can be used for MRP parameterization.