A number of experiments were conducted to study the anatomic changes in a flap following a surgical delay using the Doppler probe to add precision to the technique. After scanning the integument of a series of anesthetized animals with the probe, each was sacrificed; a total-body arterial injection was performed with a lead oxide mixture, the integument and deep tissues were radiographed separately, and the results were correlated and compared with our previous human studies. The dog was selected from the range of animals examined, and the arterial networks of a number of skin and muscle flaps were studied with and without a surgical delay. The study included the use of a tissue expander. Results revealed that an adjacent cutaneous perforator could be captured with safety on the artery at the base of an undelayed flap; that the survival length of that flap was related to the distance between perforators; that the necrosis line of the flap usually appeared in the zone of choke vessels connecting adjacent territories; that a surgical delay results in a dilatation of existing vessels with maximal effect in the zone of choke arteries; that the most effective delay was obtained by elevating the flap in stages from the base, leaving detachment of the tip until last; that tissue expansion is a form of surgical delay, with particular emphasis on vessel hypertrophy; and that similar changes occur when a muscle is delayed. The clinical applications of this investigation are presented in Part II of this anatomic review of the delay phenomenon.