1. Blood flow changes have been measured in rabbit skin homografts and autografts and an attempt has been made to correlate these changes with the presence of vasoactive agents in homogenates of the grafts. 2. During the healing-in of both types of grafts the blood flow and histamine content increased. 3. If this histamine was responsible for the vasodilatation via an H2-receptor it must have been released in the form of "nascent" or intrinsic histamine since the vasodilatation was not antagonized by antihistamine. 4. A peak of histamine concentration occurred in homografts, more than three times that in autografts at the onset of rejection. At the same time the blood flow stopped completely. 5. This vasoconstriction might be mediated by histamine since treatment with mepyramine delayed the cessation of blood flow from 5 days up to 7 to 10 days. 6. Prostaglandins appeared to be involved only in the period of peak blood flow in homografts because indomethacin did not delay the onset of rejection, but reduced the peak blood flow in the homografts at a time when there was an increased content of prostaglandin E2 in the homograft tissue.