Rats were made hypertensive by the oral ingestion of mestranol for 6 months. Carotid arteries from these hypertensive rats and from control rats were incubated in an extract of plasma from these rats, which contained 24Na and 14C-sucrose (an extracellular fluid marker). After sufficient time for equilibration, the vessels were removed and counted for 24Na; an aliquot of the incubation fluid was also counted for 24Na. At least 7 days later, after the 24Na had decayed away, the samples were counted for 14C-sucrose. By knowing the specific activity of the 24Na and the ratio of 24Na/14C in the bathing fluid, the amount of intracellular Na could be calculated for each carotid artery. The total protein content of each vessel was determined, and the results were expressed as nEq of Na per mg of vessel. The Na content of carotids from mestranol treated rats averaged 135 +/- 11 (SEM) while the carotids from the control rats averaged 125 +/- 8 nEq/mg protein. Because these values were not significantly different, this study provided no evidence that increases in arterial Na content contributed to the hypertension associated with the ingestion of mestranol in this rat model.