A production lot size model has been developed for an inventory system with deteriorating items. Both the varying and constant rate of deterioration have been included in the analysis. For the case of a varying rate, it seems impossible to obtain a simple expression for the production lot size, so a numerical method has been suggested. For the constant rate of deterioration case, an approximate expression has been derived for the production lot size. Finally, a numerical example is solved to show the impact of deterioration.