This study investigated the test–retest reliability of running economy (RE) and metabolic and cardiorespiratory parameters related to endurance running performance using a multistage incremental treadmill test. On two occasions separated by 21–28 days, 12 male middle- and long-distance runners ran at 10, 11, 12, 13, and 14 km/hr for 8 min each stage, immediately followed by a ramp test to volitional exhaustion. Carbohydrate (10% maltodextrin solution) was consumed before and during the test to provide ∼1 g/min of exercise. RE, minute ventilation ($\dot{\mathrm{V}}\mathrm{E}$), oxygen consumption (${\dot{\mathrm{V}}\mathrm{O}}_2$), carbon dioxide production (${\dot{\mathrm{V}}\mathrm{CO}}_2$), respiratory exchange ratio (RER), heart rate (HR), ratings of perceived exertion (RPE), and blood glucose and lactate concentrations were recorded for each stage and at volitional exhaustion. Time-to-exhaustion (TTE) and peak oxygen consumption (${\dot{\mathrm{V}}\mathrm{O}}_2\mathrm{peak}$) during the ramp test were also recorded. Absolute reliability, calculated as the coefficient of variation (CV) between repeated measures, ranged from 2.3% to 3.1% for RE, whereas relative reliability, calculated as the intraclass correlation coefficient (ICC), ranged from .42 to .79. $\dot{\mathrm{V}}\mathrm{E}$, ${\dot{\mathrm{V}}\mathrm{O}}_2$, ${\dot{\mathrm{V}}\mathrm{O}}_2\mathrm{peak}$, ${\dot{\mathrm{V}}\mathrm{CO}}_2$, RER, and HR had a CV of 1.1%–4.3% across all stages. TTE and RPE had a CV of 7.2% and 2.3%–10.8%, respectively, while glucose and lactate had a CV of 4.0%–17.8%. All other parameters, except for blood glucose, were demonstrated to have good-to-excellent relative reliability assessed by ICC. Measures of RE, ${\dot{\mathrm{V}}\mathrm{O}}_2\mathrm{peak}$, and TTE were reliable during this two-phase multistage incremental treadmill test in a cohort of trained and highly trained male middle- and long-distance runners.