An extensive measurement capability has been implemented in the interface message processors (IMP) of the ARPA network of computers. These measurement facilities were implemented by the network contractor, Bolt, Beranek, and Newman, Inc., to meet the measurement needs of the University of California, Los Angeles, (UCLA) network measurement center, and consist of software routines within the store-and-forward IMP to accumulate statistics, periodically record snap shots, trace selected messages through the net, and to be able to generate artificial traffic. These capabilities have been augmented by routines at the Network Measurement Center to provide selective control over the data gathering functions, as well as providing data reduction, printout routines, and more extensive artificial traffic generation facilities. The purpose of the measurement effort is to provide insight into network behavior and to support the analytic and simulation modeling work being done at UCLA. In this latter role, the measurements provide a means of verifying, or correcting, models of network behavior by performing appropriate tests in the actual network environment. The result of several such efforts will be discussed.