The paraventricular nucleus of the hypothalamus plays a pivotol role in the regulation of plasma volume. Part of the response to an increase in volume load is an inhibition of renal sympathetic nerve activity. The present experiments were designed to determine which subnuclei of the paraventricular nucleus are involved in this sympatho-inhibitory response. Experiments were performed on anaesthetised rats. Activated neurones were recognised by the expression of the early gene c-fos, identified by immunohistochemical labelling of its protein product Fos. Plasma volume loading with 4 % Ficoll 70, using an infusion-withdrawal procedure (2 ml over 1 min) repeated 15 times over 1 h revealed a total of 775 ± 101 (n = 6) Fos-positive neurones scattered throughout both the magnocellular and parvocellular subnuclei. In comparison, sustained hypertension resulted in 452 ± 56 (n = 3) Fos-positive neurones similarly distributed, whereas a normotensive control group (n = 3) displayed 115 ± 18 Fos-positive neurones. Because of this lack of a specific effect we used a more selective stimulation of right atrial receptors via a balloon placed at the junction of the superior vena cava and the right atrium so it did not impede venous return. Inflation of the balloon inhibited renal sympathetic nerve activity (36 ± 5 %, n = 7) and repetitive inflation over 1 h resulted in c-fos activation of a small number of neurones (54 ± 14) located only in the parvocellular subnuclei. Whether these are inhibitory interneurones acting within the paraventricular nucleus, or spinally projecting neurones which inhibit or excite renal sympathetic activity by an action in the spinal cord remains to be determined. Experimental Physiology (2002) 87.1, 25-32.