To examine the exponential nature of the left ventricular pressure (LVP) fall during isovolumic relaxation (IRP) in 59 patients (normal (N), 6; angina pectoris (AP), 13; myocardial infarction (MI), 24; and congestive (CCM, 6) and hypertrophic (HCM, 10) cardiomyopathies), LVP and dP/dt were measured by a Millar's catheter-tipped transducer. In P vs time relation during IRP was reasonably fitted by a straight line (r greater than 0.97) in all cases. Time constant (msec) in CCM (56.9 +/- 10.7), HCM (44.8 +/- 12.4) and MI (43.8 +/-7.4 higher (p less than 0.05) than in N (30.3 +/- 5.5), and peak (-) dP/dt (mmHg/sec) was lower (841 +/- 171, 1152 +/- 397 and 1270 +/- 211, respectively; p less than 0.05) than in N (1885 +/- 150), suggesting impaired LV relaxation in these groups. However, (-) dP/dt upstroke pattern was exponential only in both N and 8 of the AP. The (-) dP/dt upstroke in the remaining groups lost its exponential nature, showing rather a down ward-convex curvature which was especially prominent in CCM. This indicates non-exponential fall of LVP during IRP. Thus, the present results suggest that impaired relaxation disturbs the exponential nature of LV relaxation.