Sevofluran and Isofluran belong to a chemical group called halogenous hydrocarbons. Both anaesthetics are widely used in the daily medical routine to ensure life under unconscious conditions. In order to test effects of anaesthetics on respiratory rhythm we used the perfused heart brainstem preparation of adult rats (21-28d). The respiratory activity was recorded via a suction electrode placed at the nervous phrenicus (PN). Both anaesthetics were applied at concentrations from 1nM to 1mM. Changes of the frequency of phrenic nerve discharges under control conditions and after the application of anaesthetic Sevofluran (n=19) or Isofluran (n=19) were analysed. With a threshold concentration of 1nM Sevofluran, the PN discharges significantly increased 38±15%, while at a concentration of 100nM the frequency increased by 75±12%, while the results obtained with Isofluran could be divided in two equal groups: 1) no significant change in PN discharges at any concentration used and 2) significant increase of PN discharges with a threshold concentration of 10nM (115±73%) and a frequency increased of 151±79% with 100nM Isofluran. The application of Sevofluran or Isofluran in the concentration range of 1nM to 1mM had no suppressive effect on the respiratory activity, which might be an important observation in regard to their daily use as anaesthetics.