The magnitude and time course of alterations in left ventricular size, end-diastolic pressure (LVEDP), and stiffness consequent to chronic volume overloading were examined in conscious dogs. A large infrarenal aortocaval shunt was produced surgically, left ventricular dimensions were measured by cineradiography using radiopaque beads implanted subendocardially, and LVEDP was measured via an indwelling catheter. In nine experimental and seven control animals, LVEDP was related to the left ventricular end-diastolic diameter (LVEDD) over a range of LVEDP produced by transfusion and bleeding. In control studies LVEDP averaged 6.7 ± 1.42 (SEM) mm Hg, LVEDD determined at 15 mm Hg LVEDP averaged 4.8 ± 1.13 cm, and the mean slope of linear stiffness curves was 45.9 ± 2.3 mm Hg/cm. Time-dependent changes in all of these parameters were observed following creation of the shunt: A progressive increase of LVEDP reached a maximum by 2 to 3 weeks, and when early postshunt studies (mean 5.2 days) were compared with late postshunt studies (mean 6.8 weeks) LVEDP had increased from 9.7 ± 1.2 to 18.9 ± 1.1 mm Hg (P< 0.01). There was a progressive, more gradual, increase in left ventricular size, LVEDD being 4.81 ± 2.4 cm in early postshunt and 5.34 ± 2.8 cm in late postshunt studies (P< 0.01). Left ventricular diastolic stiffness showed an increase from 50.2 ± 5.6 to 96.6 ± 10.7 mm Hg/cm between early and late postshunt studies, respectively (P< 0.01), and functional compliance was reduced. In six animals the shunts were closed; at approximately 1 week there was a substantial drop in LVEDP (9.5 ± 1.9 mm Hg), and although LVEDD were unchanged stiffness decreased to 58.6 ± 12.2 mm Hg/cm (P< 0.01). Observations up to 4 months after shunt closure suggested further progressive return toward normal of these parameters. It is concluded that chronic volume overloading in the normal dog causes a time-dependent shift to the right of the diastolic left ventricular pressure-dimension relation accompanied by a progressive reduction of diastolic compliance, and that these changes are at least in part reversible following correction of the shunt.