Although changes in the extracellular matrix have been associated with changes in ventricular compliance in certain diseased states, this relationship has not been entirely established. Accordingly, we studied passive ventricular mechanics in the tight-skin (TSk) mouse, a mutant strain known to have increased cardiac collagen. In the arrested left ventricle, we determined the pressure-volume relationship and the stress-free state, as defined by the "opening angle" of an equatorial ring with a radial cut. The results showed the mean opening angle in the TSk mouse to be smaller than that in phenotypic negative controls (7 +/- 6 vs. 21 +/- 9 degrees), and there was no statistical difference in the pressure-volume curves. Histological quantification of the transmural collagen showed a uniform increase of collagen area fraction across the wall in TSk mouse, and a significantly thicker superficial epicardial collagen layer (4.68 +/- 0.87 vs. 3.34 +/- 0.76 micron). Thus, although there appears to be a decrease of residual stress in the TSk mouse heart, which may be related to the thicker epicardial collagen layer, the combination of increased myocardial collagen and the change in stress-free state did not seem to affect the passive pressure-volume relationship of the left ventricle.