A simulation, model has been developed to describe an assembly line where items are removed from a conveyor belt for processing and returned for transportation to subsequent work stations. Output characteristics are presented for the case where operator's work-time distributions can be approximated to a normal curve, and the analysis is made so as to generalize the results over a useful range of tolerance, feed interval and mean service times. In particular, the effect of buffer capacity on line performance, and its relevance to tolerance time, is studied.