Recently, the U.S. Coast Guard has established vessel traffic service (VTS) centers in four harbors. These centers are operated by watchstanders who provide an information gathering and dissemination service to mariners. As part of a program to explore ways of improving this service, computer models of watchstander interaction with equipment and communications with vessel and tug pilots were developed based on field observations and frequencies and durations of activities. The General Purpose Systems Simulator (GPSS) software package was chosen for programming and simulation of these stochastic models. These models were then constructed, programmed, simulated and verified. This technique has applications in the study of operator workload and system performance under conditions of changes in equipment, procedures and personnel.