Dynamic security assessment (DSA) provides for the study of contingencies in the transient time frame (the disturbance plus 10 seconds). The objective of DSA is to evaluate the power system with respect to those contingencies that can cause system generators to lose synchronism and separate from the rest of the system and/or lead to undamped oscillations for a group of generators. This paper describes the implementation of on-line DSA at Northern States Power Company (NSP). The main focus of this project is the implementation of the results of previous EPRI DSA projects in the on-line environment of the EMS at NSP. The purpose of the on-line DSA function is to monitor the state of the power system periodically, on demand, and upon occurrence of significant changes in the state of the power system and to ensure power system security against occurrence of contingencies.