Instrumentation for the real-time clinical measurement of pulse wave velocity (PWV) from intra-arterial pressure waveforms is presented. This time delay between pressure waveforms (obtained from two intra- arterial catheter-mounted transducers 5 cm apart) is calculated by a transputer using multiple comparisons between discrete sections of the waveforms. The method is validated by analysis of digital and analogue signals with known time delays and is used to measure changes in PWV in the right common iliac artery (RCIA) during infusions of acetylcholine (2.4, 24 and 240 micrograms ml-1) in six healthy subjects. The system measures the delay between digitally shifted triangular waveforms and pressure waveforms to a precision of about 50 microseconds, and it is superior to measurements performed by hand using a high-performance digital storage oscilloscope. When used to measure the effects of acetylcholine on the RCIA, dose-dependent reductions in PWV are recorded (-85%, -11.6%, -14.5%). It is concluded that the instrumentation enables PWV to be measured with high accuracy and precision in real time, if the pressure signals are of high fidelity and the relative amplification of the signals is carefully balanced.