Male Wistar rats were heparinized and killed with pentobarbital. The upper and lower ends of the aortae were cannulated and the blood was washed out with saline until the washings contained calcium and sialicacid-reacting material at minimal concentrations. The aortae were perfused with neuraminidase for 15 min. This caused the appearance of calcium as well as of sialic acids in the perfusate in total amounts of about 5.3 nmol and about 3.6 nmol per aorta respectively. The molar ratio of about 1.5 is sufficiently close to that determined for the association of calcium with sialic acids in vitro to suggest that their association is similar in vivo.