The effects of shock, induced with different doses of endotoxin, of vasopressors, and of vasodilators on the pressures in implanted tissue capsules (an approximation of interstitial fluid pressure), have been measured in splenectomized dogs. The pressure in these capsules is normally negative (2 to 4 mm Hg). Vasopressors caused a greater negative pressure (decrement of 0.5 to 2.5 mm Hg) which was dose related. Vasodilators (saline-phenoxybenzamine hydrochloride [Dibenzyline] and saline-phenoxybenzamine-steroids) were associated with a positive pressure (+2 mm Hg) and partially blocked the effect of vasopressors. Endotoxin shock was associated with an initial decrement in pressure (1.5 to 3.3 mm Hg) of three to five minutes followed by pressures not significantly different from control, and a terminal decrement in pressure (average, 9 mm Hg). Vasodilators were able to block the initial decrement in pressure but did not otherwise affect it.