The effect on the intrarenal vascular architecture of the graft-reaction when modified by administration of Imuran has been examined by a combination of stereomicro-angiographic and histologic methods on 14 canine renal allografts. When removed 13–140 days after the operation the grafts displayed varying micro-angiographic and histologic pictures, from largely normal morphology to a marked graft reaction with subsequent scarring and organisation or resorption. In specimens with little or no histological alterations the micro-angiographic pattern was normal, while the kidneys displaying rejection evidenced pathologic arteriovenous communications. The results suggest that because of the variability of the graft-reaction the renal grafts associated with the administration of Imuran can develop pathologic arteriovenous anastomoses with the same appearance as those seen in allografts when no Imuran has been given. On the other hand, the drug would seem to be able to retard this vascular damage, or even induce healing of these arteriovenous communication