Cardiac tissue from hamster neonates was transplanted into the hamster cheek pouch. After vascularization of the transplanted tissue the vessels were tested for their ability to convert angiotensin I (AI) to angiotensin II (AII). AI caused vasoconstriction of the coronary vessels which was reduced significantly by converting enzyme inhibitor and the AII antagonist (Sar1-Ala8) angiotensin II. It was concluded that the vessels of transplanted cardiac allografts convert AI to AII in a manner similar to that observed in the whole heart indicating that this procedure provides a good model for studying the microcirculation of the heart.