The article presents the results of an analysis of the experience of automated testing of mission-critical systems during operation, maintenance and development over 12 years. It is shown that in these processes, compared with the development process, significant changes are taking place in the goals, criteria, methods and technology of conducting functional and especially load testing of an industrially operated and accompanied automated system. The main goal of the work on testing the release is to plan and control the integrity level of the release of the system, as the main criterion for the quality and continuity of the business using an operated automated system. It is shown how automated system testing in these processes is used to assess the probability of fulfilling business production regulations using the system, that is, business continuity, as well as to assess the ability of achieving the necessary operational characteristics of the system when changing data processing technology, load, or when scaling the system. The experience of planning various types of load testing using the models given in (Pozin and Galakhov, 2011) is considered. The technology and a set of tools for automating functional and load testing developed by the authors are described.