Oxygen consumption and delivery (defined as the product of cardiac output, haemoglobin concentration and arterial oxygen saturation) and haemodynamic variables were examined in the conscious resting rat throughout the day and after the expansion of body fluid volumes. Cardiac output was measured in arbitrary units by electromagnetic flowmetry and oxygen consumption by respirometry. The variability of blood pressure in the basal state was significantly less than that of cardiac output. Oxygen consumption was significantly correlated with cardiac output and oxygen delivery. In studies undertaken throughout the day, both oxygen consumption and delivery fell in the afternoon and there was evidence that the relationship between these two variables was curvi- rather than recti-linear. During oral sodium chloride administration for 7 days, blood pressure rose and some evidence was found for an alteration in the relationship between oxygen consumption and delivery, with an excess of delivery relative to consumption, particularly on the first day of salt loading. Intravenous injection of sodium chloride solution (0.171 mol/l) did not alter the relationship between oxygen consumption and delivery. Expansion of blood volume, while the packed cell volume was maintained nearly constant, raised oxygen delivery transiently and evidence was obtained that the relationship between oxygen consumption and delivery was altered, with oxygen delivery rising relatively more than oxygen consumption. The findings are discussed in relation to the autoregulatory hypothesis of circulatory control and for the role of autoregulation in hypertensive states. The importance of relating oxygen delivery to metabolic requirements in studies of the role of autoregulation is emphasized.