In recent years, as the importance of humoral-mediated rejection has increasingly become recognized, the fact that endomyocardial biopsies (BX) evaluated according to the criteria of the International Society for Heart and Lung Transplantation often produce false-negative results has become a matter of concern. To evaluate the reliability of measuring intramyocardial ECG amplitude (IMEG) and immunofluorescence evaluation (FITC-labeled anti-IgG/ IgM staining) of endomyocardial biopsies (IFM), heterotopic neckheart transplantation (HTX) was performed on eight beagles previously sensitized through skin transplantations. After HTX, IMEG, echo, and donor-specific antibodies in serum (IgG, IgM) were determined daily and myocardial biopsies (IFM, BX) were performed once every 2 days. Accelerated (humoral) rejection occurred on the 5th (4th-5th) postoperative day and sensitivity of IMEG, IFM, and BX was 100 YO, 75 %, and 12.5 %, respectively. In each case rejection was recognized so early that it was possible to initiate therapy with "restitutio ad integrum". Our results show that, as opposed to endomyocardial biopsy (IFM, BX), IMEG diagnosis detected humoralmediated rejection early and with high reliability.