Authors carried out during one and a half year 135 anaesthesias with isoflurane. 104 of them was administered in closed, and 31 in semi-closed system. The results were compared with halothane and fentanyl anaesthesias. After brief review of the literature about isoflurane, authors evaluated their own experiences The most favourable result was the stability of the metabolic acid-base balance. One explanation of this feature is the undisturbed oxygenation of the tissues. The circulation was characterized by mild tachycardia and by transitory decrease of the systolic blood pressure. The circulatory parameters were better in closed system, than in semi-closed one, and within the former system the most stable circulation was achieved either by an empirical syringe-injection administration, or by vaporizer administration using 5 -3,8 -3,4 -3 -2,8 -2,5 -2,3 etc.% dosage schedule. The dose-related respiratory depression was of lesser degree, than with fentanyl, and was equal to that with halothane. It was found an also dose-related saving in muscle relaxants. The blood sugar level increased considerably and more prolonged, than with the control anaesthetics. The high cost of the isoflurane (and of any other inhalational agent) was partly compensated by using it in a closed system. Authors found the isoflurane--based on their own experiences and on the data of the literature--suitable for prolonged and repeated anaesthesias too, but they referred to the circumstances also, which required closer attention.