The application of queuing network theory to the performance evaluation of flexible manufacturing systems (FMS) usually requires the assumption of exponential processing times on machines. It has resulted in pessimistic, although qualitatively robust, evaluation algorithms so far. A recent computational technique, named mean-value analysis, allows to depart from this unrealistic assumption and to derive approximate algorithms for networks with general servers and FIFO service rule. The case of deterministic service times is particularly relevant for FMS's, and the corresponding heuristic technique is described and validated by means of simulation. This technique is liable to extend to the case of unreliable deterministic servers, thus enabling to assess the overall effect of short incidents on machines, on the production rate of the system.