The delay phenomenon was studied by measuring tissue oxygen tension (Psquo2) for 3 weeks in delayed flaps and normal adjacent contralateral skin in seven mongrel dogs. The Psqo2 fell after elevation of a bipedicle flap and rose again to normal by day 14. Delivery of oxygen to this flap was improved by surgical delay, so that when the bipedicle flap was reelevated on day 14 and its distal pedicle divided, minimal changes in Psquo2 values than measured in the bipedicle flap on day 0, and therefore, it too had participated in the delay phenomenon, even though only its midline edge had been incised. An anatomic explanation for the findings was sought in wounds made in 10 rabbit ear chambers. After injury, blood flow was seen to be rerouted parallel to the incision line and was increased first by vasodilation and then also by angiogeness until about day 14. Rerouting of blood by injury, inflammation, and angiogenesis due to repair appears to account for a significant portion of the delay phenomenon.