Different body positions alter hydrostatic and colloid-osmotic pressure in the limbs. The effects of the latter on exercise metabolism have not been extensively researched. We investigated the metabolic response during constant load exercise at given absolute and relative intensities, with respect to the seated (UP) or supine body position (SUP). Following University of Essex ethics approval, eight healthy individuals [mean (SD) age 19.3 (1.05) years, height 1.75 (0.08) m and mass 70.8 (5.9) kg] performed two cycling incremental load tests for the determination of peak power (PP) and the corresponding blood lactate concentration (BLC) in UP and SUP. The incremental load tests started at a workload of 50 W and increased by 25 W every 3 min until exhaustion. Subsequently three constant load tests were randomly performed lasting 20 min or until premature termination due to exhaustion. Power output was equivalent to 65% of the PP attained in the corresponding body position incremental test: A) UP-65% peak UP test; B) SUP-65% peak SUP test; C) SUP-65% peak UP test. Aerobic constant workload conditions were identified using a generally accepted definition of a steady state of the BLC (Beneke, 2003). In the incremental load test, using paired t tests, PP and BLC were significantly higher (p < 0.001) in UP (187.5 (29.8) W and 8.88 (2.23) mmol l-1) than in SUP (164 (15.3) W and 6.27 (1.6) mmol l-1), respectively. In the constant load tests, A (130 (21.3) W) and B (108 (21.8) W) showed a steady state of BLC, irrespective of pedalling rate. However, four subjects did not show a steady state of the BLC with condition C (130 (21.3) W), with 3 of these tests terminated after the 8th, 10th and 11th minute, respectively. Using one-factor ANOVA and post-hoc t-tests, the BLC at test termination was not significantly different between A (3.0 (0.16) mmol l-1) and B (3.02 (0.21) mmol l-1), however, at C a higher (p < 0.001) level of BLC (4.8 (0.53) mmol l-1) than at A and B occurred. These results indicate that at a specific power output, the body position may shift the behaviour of the BLC from a steady-state to a non-steady state, changing the working condition from essentially aerobic to partly anaerobic. Consequently this needs to be considered for exercise testing where specific body positions are required such as in many clinical settings.