Pressure overload left ventricular (LV) hypertrophy (LVH) induces ventricular dysfunction during stress, which is commonly attributed to diminished myocardial capillary density and ischemia. Immature hearts with LVH have a normal coronary flow reserve and capillary density. The purpose of this study was to determine 1) whether young lambs with LVH had an abnormal response to chronotropic stress, 2) whether nonischemic mechanisms contributed to the abnormal response, and 3) whether the age at which LVH was induced affected the response. We assessed LV endomyocardial function, perfusion, and Ca(2+)-adenosinetriphosphatase (ATPase) mRNA levels in chronically instrumented lambs with and without LVH and adult sheep with and without LVH. Rapid pacing induced diastolic dysfunction, increased time constant of isovolumic relaxation using an iterative fit (tM), and elevated LV diastolic pressures in young lambs and adult sheep with LVH. During pacing, tM was greater in the adult sheep with LVH than in the young lambs with LVH. Ca(2+)-ATPase mRNA levels were 79% less in adult sheep with LVH than in those without. Ca(2+)-ATPase mRNA levels in lambs with and without LVH and adult sheep without LVH were similar. Diastolic dysfunction occurred in the absence of subendomyocardial hypoperfusion, suggesting a nonischemic mechanism. In adult sheep with LVH diastolic dysfunction was associated with a marked reduction in Ca(2+)-ATPase mRNA levels.