The time constant (T) is commonly used for representing the isovolumic fall in left ventricular pressure (LVP) because the isovolumic fall is monoexponential under most circumstances. However, the determination of T is cumbersome due to the requirement of a set of LVP versus time values in fitting a monoexponential function. RT64, which is the time from the minimum value of the time derivative of LVP (-dp/dtmax) to 36% of -dP/dtmax, can be derived beat-by-beat, on-line, and may be a simple alternative to T. Through the use of 240 pairs of values derived from six conscious instrumented dogs treated with different cardiovascular active agents, RT64 and T were found to be significantly correlated (r = 0.835, p less than 0.05) according to the equation RT64 = 0.725T + 10.2. In conclusion, RT64 may provide a simple alternative to T in representing the isovolumic relaxation process of the left ventricle.