To evaluate relaxation mechanics in the wall of the left ventricle needle mounted miniature pressure transducers were inserted at the subendocardial (ENDO) and subepicardial (EPI) level of the anterior wall of the left ventricle during acute open-chest experiments in 10 mongrel dogs. Pressures were recorded during control, volume load and after verapamil administration. The relaxation time constant (τ) was determined by fitting a monoexponential with offset to the isovolumic relaxation period of the ENDO, EPI and left ventricular pressure (LVP) tracings: p = p₀e<sup>-t/τ</sup> + p<sub>1</sub>. Mean τ-values for LVP, ENDO and EPI during control were (mean ± 1 SD, ms): 38 ± 5, 60 ± 12, 83 ± 5; during volume overload: 55 ± 10, 72 ± 20, 85 ± 31 and after verapamil administration: 58 ± 13, 60 ± 17 and 73 ± 15, respectively. Relaxation time constants of ENDO and EPI were significantly longer than those of LVP during control and volume loading but not after verapamil when only EPI was significantly different from LVP. These results demonstrate that relaxation indices obtained from LVP may not always reflect intramyocardial mechanics.