One of the largest items of expense in crop production is the cost of owning and operating farm machinery. Undoubtedly, machinery cost varies widely from one farm to another. Cost per acre, per hour, and per unit of production also varies widely among farms. This variation is in part a result of differences in size of farms and consequently in the volume of production over which fixed costs can be spread. Also, part of the variation is due to the success with which individual farmers adapt their use of machinery to the particular needs of their farms. The purpose of the study reported here was to estimate the cost, including labor cost, of owning and operating tractors and the 13 major farm implements used on farms in northwestern Ohio, and to show how this cost was affected by the size of farms. The report presents estimates of cost per unit of use on three sizes of farms. This is one of a group of studies by the Economic Research Service on cost of production by size of farm in several major type-of-farming areas