1. We wished to determine whether, during pregnancy, there is reduced renal response to atrial distension, whether secretion of atrial natriuretic factor (ANF) is suppressed and whether neural input from the atrial volume receptors to the central nervous system is altered. 2. Conscious, chronically instrumented female rats were used. Atrial distension was achieved by implanting small balloons positioned at the superior vena caval/right atrial junction; inflation of the balloon did not impede blood flow through the heart. 3. In virgin rats, atrial stretch caused increased urine volume, urine sodium and potassium output and decreased free water clearance. These responses were abolished during pregnancy. 4. In response to atrial stretch, plasma ANF levels increased significantly in virgin rats. No such secretory response was observed in the pregnant animals. 5. Distension of isolated atria derived from unmated and 7 day pregnant rats resulted in an increase in secretion of ANF into the perfusate. Atria from 14 and 21 day pregnant rats were unresponsive to distension. 6. Pretreatment with oestradiol (50 micrograms daily for 10 days) caused plasma ANF levels and ANF secretion by isolated perfused atria to increase. By contrast, testosterone pretreatment (15 mg twice weekly for 2 weeks) abolished stretch induced secretion of ANF by isolated atria. 7. C-fos activity in the paraventricular nucleus of virgin rats increased in response to atrial distension. This response was reduced in the 7 day pregnant rats and abolished at 21 days. 8. We conclude that there is attenuation of both hormonal and neural responses to atrial distension in the pregnant animal. This allows blood volume to increase without eliciting homeostatic mechanisms to eliminate the extra fluid.