The role of the paraventricular nucleus (PVN) of the hypothalamus in the cardiovascular response induced by blood volume expansion was examined in anaesthetised rabbits, in which simultaneous recordings were made from a renal sympathetic nerve and one of other sympathetic nerves, the inferior cardiac nerve, a splanchnic nerve and an adrenal nerve. Activation of PVN neurones, by discrete injections (25-100 nl) of d,l-homocysteic acid (DLH, 0.2 M) produced one main pattern of sympathetic nerve activity accompanying a pressor response (57-86 % of PVN sites). This was a decrease in renal sympathetic activity (27 +/- 12 %) and an increase in splanchnic (60 +/- 12 %), adrenal (31 +/- 9 %) and cardiac (42 +/- 8 %) sympathetic activity. Sites in the PVN from which these combinations of nerve activity were obtained were not confined to a specific subnucleus. An increase in renal sympathetic activity which was reversed to a decrease by reducing the volume of DLH injected was obtained at 10 sites in the PVN. These sites were mainly located in the dorsal parvocellular subnucleus. Varying combinations of sympathetic activation were obtained at a minority of sites. It is concluded that the PVN can evoke a differential pattern of sympathetic discharge which may be functionally significant in the control of blood volume regulation, as it mimics that seen on acute volume expansion.