We explored whether the blood vascular system of the chick embryo adapts its structure to meet the maximum or average oxygen needs of the tissue cells. Chick embryos were grown in continuous 12% oxygen, continuous 16% oxygen, and intermittent 12% oxygen in which the embryos were exposed to 12% oxygen for 4 h each day. Control groups were grown in room air. Measurements of structural vascular resistance (SVR), i.e., the resistance of the maximally dilated vasculature, were used to estimate the whole body vascularity of the 14- or 15-day-old embryos. Continuous exposure to 12% oxygen decreased SVR by 63.1 +/- 1.2 (SE) %, and intermittent exposure to 12% oxygen decreased SVR by 55.6 +/- 0.5% when compared with a 15-day-old normoxic control group. Based on studies with continuous exposure to different levels of low oxygen, it was predicted that exposure to 19.5% oxygen, the average concentration for the intermittent hypoxia group, would decrease SVR by 15.0 +/- 0.3%. These results indicate that intermittent hypoxia at 12% oxygen was approximately 90% as effective as continuous hypoxia at the same level in decreasing SVR and about four times more effective than 19.5% continuous oxygen. Therefore, the results support the hypothesis that the blood vascular system adapts its structure to meet almost entirely the maximum oxygen needs of the tissue cells.