The 'fast flush test' is commonly used to determine the dynamic response of catheter-manometer systems in vivo. The authors compared the fast flush test with an established bench top test in vitro. The results of the two tests were compared in fifteen underdamped radial artery catheter-manometer systems. There were significant differences between the results, and the variance of estimates within measurement systems was significantly greater in the case of the fast flush test. Nevertheless, the differences between the tests were small by comparison with the errors associated with either test; in addition there was a significant linear correlation between the results of the two tests. In the in vitro situation, therefore, the fast flush test provides a crude estimate of the dynamic parameters of underdamped catheter-manometer systems.