Freshly isolated thoracic aortic endothelial cells from 4- and 12-month-old male rabbits were exposed to two periods of increased flow rate and the levels of ATP, endothelin (ET) and arginine vasopressin (AVP) released into the perfusate were measured. The rapid, significant release of ATP and ET with increased flow from cells isolated from 12-month-old rabbits was not seen when cells isolated from 4-month-old rabbits were exposed to the same conditions. Increased flow did not stimulate AVP release from cells from either age group. Basal release with low flow also differed in endothelial cells from 4-month-old compared to 12-month-old rabbits; thus in 4-month preparations basal release of AVP was greater, that of ET was smaller, while that of ATP was unchanged relative to 12-month preparations. Immunohistochemical studies showed that almost half as many endothelial cells isolated from 4-month-old rabbits were immunoreactive to ET and AVP compared to cells isolated from 12-month-old animals. It is suggested that the endothelial contribution to the control of vascular tone in rabbit thoracic aorta changes during ageing.