BACKGROUND The postoperative care of patients subjected to cardiac surgery frequently require a complete recovery with intravenous fluids, but crystalloid solutions like normal saline may increase the interstitial oedema, and it is also well known that fluid overload increases mortality. OBJECTIVE To compare the effect of 7.5% hypertonic saline (HS) with 0.9% normal saline (NS) on lactate clearance, as well as the haemodynamic response of patients during the first day after cardiovascular bypass surgery. METHODS The study included patients 18 years of age and older with coronary artery disease and/or heart valve disease, and who underwent bypass surgery and/or cardiac valve replacement and were randomly assigned to receive 4mL/kg of HS or NS intravenously for 30min once they were admitted to the ICU. Lactate, arterial blood gases, heart rate, central venous pressure, and pulmonary wedge pressure were measured at 0, 6, 12, and 24h after being admitted to the ICU. The analyses were carried out with an intention-to-treat principle. RESULTS Out of a total of 494 patients evaluated, 102 were included and assigned to the HS groups (51 patients) or NS (51 patients). The mean age of the participants was 59±14 years, and 59.8% were male. No statistically significant differences were observed between two groups in the lactate clearance, or in any of the secondary outcomes. CONCLUSIONS Our study failed to show a better lactate clearance in the group on hypertonic saline, and with no evidence of a higher incidence of adverse effects in that group.