The effects of vasoactive substances on mean circulatory filling pressure (MCFP), an index of total body venous tone, were determined in conscious rats. Cumulative doses of saline (0.9% w/v NaCl solution), methoxamine (alpha 1-adrenoceptor agonist), B-HT920 (alpha 2-adrenoceptor agonist) noradrenaline and vasopressin, and individual doses of angiotensin II (AII), were infused into the rats. Mean arterial pressure (MAP), MCFP and heart rate (HR) were determined before and during the plateau responses to infusions of the vasoactive substances. The infusions of all the agonists caused a dose-dependent increase in MAP and a decrease in HR. The infusion of saline affected neither MAP nor HR. The infusions of saline and methoxamine did not affect MCFP while the infusions of B-HT 920, noradrenaline and AII increased MCFP. MCFP was slightly increased during the infusion of high doses of vasopressin. It was concluded that receptors for the alpha 2-adrenoceptor agonist and AII are involved in the control of venous tone. Receptors for the alpha 1-adrenoceptor agonist and vasopressin are not important for the control of venous tone.