- In this paper, we examine throughput (mean number of completed assemblies per unit time) of closed assembly type queueing networks where machine processing times are drawn from general distributions. The system dynamics are characterized via a set of stochastic difference equations; it is shown that the system state can be modeled by a discrete index Markov chain on a continuous state space. Standard Markovian analysis is employed to derive an approximate expression for system throughput, following discretisation of state space. Four examples of CONWIP (CONstant Work IN Process) systems are given that illustrate the results. 1999 Published by Elsevier Science B.V. All rights reserved.