Electrical impedance tomography (EIT) is a recent imaging technique based on electrical impedance, offering the possibility of measuring pulmonary perfusion. In the present study the influence of several pulmonary haemodynamical parameters on the EIT signal were investigated. First, the influence on the systolic wave of the EIT signal (delta Zsys) of stroke volume, large pulmonary artery distensibility (both assessed by means of MRI) and the extent of the pulmonary peripheral vascular bed in 11 emphysematous patients (reduced peripheral vascular bed) and 9 controls (normal peripheral vascular bed) was investigated. Second, the influence of hypoxic pulmonary vasoconstriction on delta Zsys was examined in 14 healthy subjects. Finally, the origin of the diastolic wave was examined in three patients with atrioventricular dissociation. Multiple regression analysis showed that delta Zsys was only dependent on the variable emphysema (p < 0.02), but not dependent on stroke volume (p < 0.3) or pulmonary artery distensibility (p > 0.9). The mean value of delta Zsys for emphysematous patients (131 +/- 32 arbitrary units (AU)) was significantly lower (p < 0.001) than in the control group (200 +/- 39). In the group of healthy subjects delta Zsys decreased significantly (p < 0.001) during hypoxia (193 +/- 38 AU) compared with rest measurements (260 +/- 62 AU). The absence of the diastolic wave in the cardiological patients suggests the influence of reverse venous blood flow on the EIT signal. It is concluded that volume changes in the small pulmonary vessels contribute significantly to the EIT signal. Moreover, the hypoxia induced decrease in delta Zsys indicates the potential of EIT for measuring pulmonary vascular responses to external stimuli.