Although rats are commonly used to study left ventricular (LV) hypertrophy, measurement of LV mass and dimensions has required killing the rat. To determine the accuracy of echocardiography in rats, blinded crossectional area (CSA) and LV mass measurements using either the cube function (LVM) or an elliptical model (LVMel) from high resolution M-mode echocardiograms were compared to necropsy LV weight (0.28 to 1.5 g), in 41 normotensive (body weight 116 to 762 g) and 17 hypertensive rats (350 to 560 g). Postmortem chamber volumes in 28 normal rats (0.02 to 0.19 mL) were also compared to echocardiographic volumes derived from the elliptical model. Correlation with LV weight was r = 0.87 for LVM, 0.90 for CSA and 0.93 for LVMel (all P less than .00001). Comparison of hypertensive and body-weight-matched normotensive rats revealed the upper normal limit for both LVMel and CSA to have 89% sensitivity and 100% specificity for detection of post mortem LV hypertrophy. Necropsy LV volumes were more closely related to systolic echocardiographic volumes than to diastolic volumes (r = 0.78 v 0.71, both P less than .00001), compatible with the effects of post mortem contracture. Stroke volume determined invasively in 5 Wistar rats by thermodilution was similar to that obtained using elliptical model echo volumes in 5 rats of the same body size (0.35 +/- 0.05 v 0.30 +/- 0.06 mL/beat). Echocardiography can be used to evaluate LV structure and function in rats and to detect in vivo LV anatomic differences induced by hypertension.