1. Beat to beat changes in cardiac output during standardized Valsalva's manoeuvres were recorded using electrical bioimpedance cardiography in 13 normal subjects. 2. Cardiac output increased by 12 ± 5% after 1 s of straining solely because of an increase in heart rate. Subsequently, cardiac stroke volume and cardiac output fell during the strain to lows of − 40 ± 6% and − 21 ± 3% respectively at 15 s. 3. There was a sustained increased in cardiac output (maximum + 17 ± 4%) in the late post strain period. 4. The mean coefficient of variation in impedance measurements of cardiac output was 6.8% during all parts of Valsalva's manoeuvre, with no single value exceeding 10%.