This report describes a simulation of adults' retrieval of arithmetic facts from a network-based memory representation. The goals of the simulation project are to: demonstrate in specific form the nature of a spreading activation model of mental arithmetic; account for three important reaction time effects observed in laboratory investigations; and provide a basis from which developmental changes in performance can be meaningfully understood. The simulation, based on the process of spreading activation among related nodes, retrieves correct answers as the activation from a problem's addends/multipliers intersects in the network. Two decision processes are simulated. The first consists of an attempted matching of retrieved and stated answers, essentially a process of number inequality judgments. The second decision mechanism involves competition among related nodes, based on levels of activation generated during the search phase for both retrieved and stated answers. Both decision mechanisms are necessary to predict the separate effects of split and confusions which have been observed in the laboratory. The conclusion of the paper discusses the relationship of the simulation to developmental processes in mental arithmetic performance, considering the interplay between declarative knowledge of the sort simulated and "procedural" knowledge of arithmetic algorithms and heuristics. (Author/JN)