The rat tail contains a dorsal vein ... of gonadotropins and prolactin in ovariectomized rats. Endocrinology 90(3), 707–715 (1972). McGuill, M.W. & Rowan, A.N. Biological effects of blood ...
If the rat does not resist, the investigator can now insert the catheter. Otherwise, it is critically important to wait until the local anesthetic is fully effective, and the tail no longer moves.