Different possible strategies for the replacement of tractors are empirically compared with each other for different tractor sizes, taxation rates, interest rates and annual use of the machine. Actual data for the period 1973-1989 were used. Results show that the optimum replacement age is not only dependant on the individual level of each of the above-mentioned variables, but also on how the variables vary together. The magnitude of each variable thus influences the effect of the other variables on the optimal replacement decision.