Introduction: Metabolic flexibility (MF) is the ability of individuals to switch between energy substrates in response to changing physiological conditions (1), being affected by factors such as sex, age and training level (2). For instance, men reported 28% higher Maximum Fat Oxidation (MFO) compared to women (2). Age could also mediate this process due to a higher energy expenditure at rest as well as a poor mitochondrial function with aging, leading to higher lactatemia for a similar load and limiting the uptake of free fatty acids (1). Finally lower levels of training may also affect MF since sarcodynapenia and reduced insulin action may compromise lipid metabolism. This study aims to analyse the behaviour of MF in active older women by means of fatty acid oxidation (FATox) and carbohydrate oxidation (CHOox) values. Methodology: Nine older women who met the inclusion criteria (> 60 years and moderately active) started this pilot study. 7 participants (73±7.23 years, 64±24.18 kg, and 30±11.33kg of muscle mass) completed the protocol. Participants performed an incremental cycling test which started at 30w and increased progressively 10w each three minutes until volitional exhaustion or technical criteria: visual analogue scale of Pain (VAS) >5; rating of perceived effort (RPE) >7; respiratory exchange ratio (RER) >1.1.; and/or arterial oxygen saturation (SpO2) <92%. Gas exchange, heart rate (HR) and power output completed these outcomes which were continuously recorded during the test, in addition to lactate pre- and post-test (3-5min). FATox and CHOox calculation was performed by indirect calorimetry, analysing oxygen uptake (VO2) and carbon dioxide production (VCO2). From these data, Frayn's equations (3) were applied, taking the values of the last 60s of each level of intensity (4) with the assumption that urinary nitrogen excretion was 0, as previously stated (1). Results: Table 1 and Table 2 show the main descriptives, accompanied with an increase in blood lactate (BLabaseline: 1.4±0.4 mmol·L-1; BLa3min 5.87±3.33 mmol· L-1; BLa5min 4.87±2.8 mmol· L-1). Conclusions: Preliminary results reflect a remarkable metabolic inflexibility in older women, which is possibly a consequence of poor mitochondrial function (1), as supported by current literature (5). Moreover, it appears that their MFO (0.10±0.11 g·min-1) could correlate with greater mitochondrial dysfunction in this population with respect to others with greater literature, i.e., obese or sedentary middle-aged, with markedly higher values: 0.36±0.12 g· min -1 and 0.24±0.03 g·min-1, respectively (2).  On the other hand, Table 2 displays that these participants reach their MFO at the relative power output of 60%, higher than moderately active middle-aged by San-Millan and Brooks (1). More than a remarkable improvement in MFO, it could be due to the short number of periods completed in our sample (100% very closed to the test starting intensities). In addition, current literature (3) already suggest that shorter periods of 10min, between 3 and 4 min, may be more appropriate for assessing MF in unfit population, due to the limited benefits of longer periods and the associated fatigue that may limit the performance of the test in populations with mitochondrial dysfunction.