The reliability of impedance cardiography for absolute and relative measurements of stroke volumes is assessed by comparing data with electromagnetic flowmeter measurements in 14 anaesthetized mongrel dogs. Cardiac output was altered in each dog (as much as possible) by a variety of physical and pharmacological interventions. Impedance cardiography does not provide absolute values for stroke volumes, although relative changes may be calculated with reasonable reliability if a correction factor ('CIMP') is employed. 'CIMP' is around 0 . 8 in subjects breathing spontaneously or when alternative pressure ventilation is applied. However, this factor can only be used in an intermediate range of stroke volumes (10-30 ml in the dog). At lower and higher cardiac output levels other correction factors must be observed accordingly. Intermittent positive pressure ventilation increases the sensitivity of relative stroke volume measurements by transthoracic impedance, leading to an average 'CIMP' of 1 . 5. For calculation of relative stroke volume changes by impedance cardiography, the simple formula: (formula; see text) is satisfactory instead of the more complicated Kubicek formula.