We try to compare an environment in different types of hospitals. The University Hospital iD. Prague is located in the centre of the town in historical buildings. Homolka Hospital in Prague is one of the newest monoblock hospitals, well equipped; smaller but specialised (e.g. Leksell Gamma.Knife). We compared tbe concentrations of anaesthetic gases and temperature in operating theatres with or without air-condition. The effect of ventilation systems was confirmed, when the systems were good operated.