The closing valves are the main part of time in stand-by state. It is characteristic that the reliability of the valves decreases during stand-by time. Therefore the valves are tested periodically in order to check their condition. On the other hand, the testing may cause failures and thus decrease the availability of valves. Through a proper choice of test internal these competing factors can be balanced against each other. Optimum tests interval of closing valves is considered with the aid of simple unavailability models and using reliability data collected from Finnish BWR plants.