implication of certain forms of depreciation on machinery repair functions are explored. Inconsistencies between farm machinery repair functions in the Agricultural Engineers Yearbook and commonly accepted depreciation schedules are presented. Accurate machinery repair cost estimates are a prerequisite for a variety of decisions common in farm management. Machinery replacement, new versus used machinery purchase decisions, lease versus own decisions, and cost of production estimates are dependent upon repair cost. objective of this paper is to discuss conditions imposed upon the mathematical form of machinery repair function by generally accepted depreciation schedules. Repair functions presented in the Agricultural Engineers Yearbook [1] will be investigated to determine if the conditions are violated. Castle et. al., [2] state, The amount of depreciation charged should correspond to the loss in value of the asset over time. Furthermore, Castle et. al., go on to indicate that three commonly accepted depreciation methods are straight line (SL), sum of years digits (SD) and declining balance (DB). These depreciation methods were also legally acceptable for income tax purposes' until 1981 when depreciation will be computed using a capital recovery factor which is based upon a combination of the declining balance and the straight line method. In mathematical form, these depreciation methods imply D(t) > 0 Condition 1 D(t)/At 0 Condition 3 where D(t) is the depreciation or loss in machine value at machine age t. Condition 1 implies that depreciation will be positive or that the machine loses value with age. Presumably the machine will be used and total accumulated usage will increase with age so Condition 1 could be restated in terms of usage. Eventually usage is expected to wear out a machine, and so the more a machine is used, the less remaining use is embodied in the machine resulting in declining value and positive depreciation. Of course unusual maintenance strategies or unexpected changes in prices of commodity produced by machinery may invalidate Condition 1. Condition 2 simply states that depreciation is either constant (straight line) or declining (i.e., declining balance or sum of years digits) and that if depreciation is declining it is declining at a decreasing rate (i.e., declining balance) or constant rate (i.e., sum of years digits). Agricultural Engineers Yearbook [1] specifies remaining value (used price) functions as a portion of list (new) price in the following mathematical form