SUMMARY To investigate the role of circulating humoral substances in the patbogenesis of increased vascular wall water and sodium concentration in experimental hypertension, rabbit aortic media explants were cultured in tissue culture medium supplemented (10-20%) with serum obtained from the same dogs (n = 7):) before the induction of hypertension; 2) after wrapping one kidney in silk (two-kidney perinephritic hypertension, 2-KPHT); and 3) after contralateral nephrectomy (1-KPHT). Cultures also were run with serum of sham-wrapped and then unilaterally nephrectomized nonnotenshe dogs (n = 4). After 3 weeks of culture, the explants were harvested, and their water, sodium and potassium concentration was measured. Compared to the composition of explants cultured in prehypertensive serum, the water concentration of explants cultured in 1-KPHT and the sodium concentration of explants cultured in 2-KPHT and 1-KPHT serum were increased (p < 0.05). The water and electrolyte content of explants cultured in sera of sham-operated normotensive control dogs was the same regardless of the type of serum used, preor post-shim surgery or post-nephrectomy. The effects of serum from hypertensive dogs were not explained by variations in serum creatinlne, sodium and potassium levels or in plasma renin activities. The experiments provide evidence for the role of serum factoids) in the pathogenesis of abnormal vascular wall water and sodium concentration in experimental hypertension.