An analytical model for the approximate evaluation of the performance of priority schemes in the IEEE and SAE token bus protocols is developed. The model determines the average date latency of the four-level priority scheme in a token bus protocol. The priority scheme is analyzed in the framework of a single service system, where a station can transmit only one message each time it captures the token. From the analytical model, the relationship between the network parameters and network system performance can be determined for each priority level. The analytical results have been verified by simulation experiments.