The authors report the results of an experimental study on distributed simulation of two open queuing networks. The distributed simulation scheme considered is a simple variation of the scheme given by K. M. Chandy and J. Misra (1979) using NULL messages. The authors present a novel approach to study the relationship between the overhead and performance of a distributed simulator, and illustrate the approach by studying these two example networks. Moreover, the authors define and measure two measures of ideal speedup of distributed simulation over sequential simulation. These values of ideal speedup are much less than simply the number of processors, and hence provide a more realistic value for the ideal speedup.