Abstract Objectives. The aim of this study was to assess 1) whether quantitative ultrasound tissue analysis by serial measurements of myocardial echo amplitudes can detect and monitor the onset and degree of acute cardiac rejection, as well as its resolution of acute rejection during treatment, and 2) whether changes in myocardial echo amplitudes are modified by repeat additional rejection episodes. Background. Previous experimental studies, all involving heterotopic heart transplantation, have consistently shown reproducible alterations in myocardial echo amplitude during acute rejection episodes untreated by immunosuppressive agents. Methods. Two-dimensional echocardiographic long-axis views were obtained daily under strict standardization in 12 dogs after heterotopic cervical heart transplantation (mean survival time 16.1 days) and digitized into a 256 × 256 × 8 matrix. Myocardial echo amplitudes were analyzed by gray level histogram statistics in regions of interest (45 × 12 pixels) within the proximal septum and posterior wall and correlated with the results of daily transmural myocardial biopsies. Maintenance immunasuppressive therapy consisted of cyclosporine, azathioprine and steroids. Additive steroids were given during acute cardiac rejection. Results. All dogs experienced at least one moderate or severe episode of acute cardiac rejection. Successful resolution and repeat acute rejection were observed in three dogs. On 65 days, the left ventricular biopsy specimens showed no evidence of acute rejection. Mild acute rejection was present on 36, moderate on 29 and severe rejection on 40 days. End-diastolic mean (± SD) gray level increased progressively from 100.7 ± 20.4 for no acute cardiac rejection to 113.8 ± 23.1 for mild rejection (p = NS vs. no rejection) to 126.0 ± 16.1 for moderate rejection (p < 0.01) and to 136.3 ± 12.6 for severe rejection (p < 0.01), In each individual dog, a correlation between daily measurements of mean gray levels and histologic cardiac rejection grades was found (r mean = 0.80 ± 0.14 [range 0.57 to 0.97], n = 12). In three dogs with transient complete histologic resolution of acute cardiac rejection, mean gray level did not return to values before rejection (108.0 ± 15.4 vs. 87.2 ± 8.4). The subsequent second episode of rejection was characterized by higher gray level values than those associated with the first rejection episode (141.3 ± 14.4 vs. 124.3 ± 20.9). Conclusions. Acute cardiac rejection is associated with a progressive increase in mean gray level. Changes in myocardial echo amplitudes in individuals may thus prove a useful tool for the noninvasive detection and monitoring of acute rejection. Increased mean gray level values after resolution of rejection may indicate persistent structural tissue abnormalities after rejection and demonstrate the need to define new baseline values after histologic resolution of an acute rejection episode.