In experiments on 51 healthy anaesthetized and paralyzed rabbits the changes in parameters of mechanics of breathing during high frequency jet ventilation (HFJV) were determined and the mechanisms responsible for these changes were investigated. In the first series of experiments with two groups of animals ventilated by HFJV with relative inspiratory time ti = 0.5 and ti = 0.7 airway resistance (Raw) after 5 h of HFJV in the ti = 0.5 group increased from 1.14±0.05 to 2.31±0.09 kPa.l_1.s (P<0.001), in the ti = 0.7 group from 1.22±0.04 to 1.78±0.08 kPa.l-1.s (P<0.01). Dynamic compliance (Cdyn) decreased in the ti = 0.5 group from 0.041 ±0.004 to 0.017±0.001 l.kPa-1 (P<0.01) and in the ti = 0.7 group from 0.034± 0.003 to 0.022± 0.002 l.kPa-1 (P<0.01). In the second series of experiments a group of animals was ventilated by HFJV after cervical vagotomy. The deterioration of Raw and Cdyn was significantly reduced in vagotomized rabbits in comparison to the controls without vagotomy. Finally, the study of phospholipid content in bronchoalveolar lavage fluid revealed no significant differences after 5 h of artificial ventilation or spontaneous breathing. These data indicate that HFJV results in changes in the parameters of mechanics of breathing in healthy lungs, which may be attenuated, but not fully eliminated, by bilateral cervical vagotomy. The decrease in Cdyn and increase in Raw are probably not due to changes in the pulmonary surfactant content.