Responses of the reproductive vascular system during the egg-formation cycle of unanaesthetised laying hens.

1. Vascular adjustments to the process of egg formation were examined in the unanaesthetised laying hen, by the radioactive microspheres method. 2. Three- to four-fold increases in blood flow were found in segments surrounding the egg during its passage along the oviduct, possibly due to an enhanced metabolic activity in the muscle layer of the oviduct. 3. Shell-gland blood flow was minimal in the absence of an egg and increased gradually to a maximum (5-fold) about 5 h after entrance of the egg into the shell gland. This parallels the rate of calcification of the egg shell. 4. Changes in blood flow in the ovarian follicles and other parts of the oviduct were small while the egg was in the shell gland. This might be typical for the reproductive system of the fowl, which undergoes little structural alteration during egg formation.