Use of micro-conductance pressure-volume (PV) catheters is a standard way to assess load-dependent and load-independent indices of left ventricular (LV) function in vivo. Catheterisation of the left ventricle is routinely performed via either a carotid artery approach or a thoracotomy followed by direct apical stab. While performance of a thoracotomy necessitates mechanical ventilation, the carotid approach can be performed using either mechanical or spontaneous ventilation. These two main surgical approaches are important because they provide flexibility for a variety of animal models of cardiac disease. However, it is unclear whether differing techniques can lead to altered haemodynamic parameters in rats. In order to investigate this, PV measurements were performed in three groups (1-3) of adult Wistar rats (312±7.7g; mean±S.E.M.) each undergoing a different technique: (1) thoracic approach with mechanical ventilation, (2) carotid approach with mechanical ventilation or (3) carotid approach with spontaneous ventilation. A (1.9F) Scisense catheter was used in conjunction with a Scisense control box with in-built volume calibration (FV896B). A tracheostomy followed by tracheal cannulation permitted mechanical ventilation (68 breaths.min-1, 2.4ml volume, Harvard Apparatus 683) for groups 1 & 2. Spontaneous breathing was permitted via a face mask for group 3. Anaesthesia was induced and maintained using isofluorane (1.5-2%) in all three groups. Parallel conductance (Pc) (to account for the applied current flowing into the surrounding tissues) was assessed using a 30µl bolus of 15% hypertonic saline. A number of haemodynamic parameters were assessed and those which were significant between groups are shown in Table 1 (ANOVA, P<0.05). These data demonstrate that systolic and diastolic function were increased when using the carotid approach with spontaneous ventilation vs. thoracic approach with mechanical ventilation. In addition to changes in pressure, a rightward shift in volume parameters was present in animals undergoing carotid approach with mechanical ventilation vs. carotid approach with spontaneous ventilation. This data set provides a reference guide for mean LV parameters in rats during the three commonly used approaches for PV measurements. While these three techniques may be used to measure haemodynamic parameters in different models of cardiac dysfunction, the variation attributable to each technique should be taken into account.