Study objective - The aim was to study the effects of changes in ventilation variables on cardiac function during high frequency jet ventilation. Design - Controlled changes in mean airway pressure (2-10 cmH2O), tidal volume (4 an 8 ml), and frequency (4-8 Hz) were used to assess the effects of ventilatory variables on cardiac output in cats with normal and reduced lung compliance during high frequency jet ventilation. Cardiac output was continuously measured with an electromagnetic flow probe placed around the aorta. Respiratory compliance was reduced by lung lavage. Experimental material - Seven anaesthetised adult cats, weight 3.0(SEM 0.2) kg, were used in the study. Measurements and main results - Cardiac output fell as mean airway pressure was raised from 2 to 10 cm H2O at all frequencies and tidal volumes. At identical levels of minute ventilation and mean airway pressure, cardiac output was improved with combinations of smaller tidal volumes and higher frequencies. Reducing respiratory compliance by lung lavage dampened the effect of mean airway pressure on cardiac output, but maintained the relationship between ventilatory variables observed before lavage. Conclusion - At identical settings of minute ventilation and mean airway pressure, cardiac output may be improved with smaller tidal volumes during high frequency jet ventilation.