The workload represents a factor which acts burdensome on human organism and, depending on its level affects well-being, or leads to disease or injury. As a result, there is a distortion in optimal working relationship in the system man – machine – environment, which may be reflected in the reduction of productivity and efficiency throughout the system. The contribution deals with the issue of the effects of stress and psychological overload at work, which affects significantly the performance of employees. The article describes the basic legislation regarding the workload in conditions of the Slovak Republic, it describes the types of loads, different causes, manifestations and consequences of load. According to theoretical knowledge about stress and loads, the questionnaire method was applied for assessment of mental workload in terms of working conditions level in the unnamed service organization.