To determine the effects of creatine supplementation on cardiorespiratory responses during a graded exercise test (GXT) 36 trained adults (20 male, 16 female; 21-27 years old) performed two maximal GXTs on a cycle ergometer. The first GXT was done in a nonsupplemented condition, and the second GXT was done following 7 days of ingesting either 5 g creatine monohydrate, encased in gelatin capsules, four times daily (CS, 13 male, 6 female), or the same number of glucose capsules (PL, 7 male, 10 female). CS significantly (P<0.05) improved total test time [pre-CS = 1217 (240) s, mean (std. dev.) versus post-CS = 1289 (215) s], while PL administration had no effect (P>0.05) on total test time [pre-PL= 1037 (181) s. versus post-PL= 1047 (172) s]. In addition, both oxygen consumption (VO2) and heart rate at the end of each of the first five GXT stages were significantly lower after CS, but were unchanged after PL. Moreover, the ventilatory threshold occurred at a significantly greater VO2 for CS [pre-CS = 2.2 (0.4) l x min(-1) or 66% of peak VO2 versus post-CS = 2.6 (0.5) l x min(-1) or 78% of peak Vo2; pre-PL = 2.6 (0.9) l x min(-1) or 70% peak VO2 versus post-PL = 2.6 (1.1) l x min(-1) or 68% of peak Vo2]. Neither CS nor PL had an effect on peak Vo2 [pre-CS = 3.4 (0.7) l x min(-1) versus post-CS = 3.3 (0.7) l x min(-1); pre-PL = 3.7 (1.1) l x min(-1) versus post-PL = 3.7 (1.1) l x min(-1)]. Apparently, CS can alter the contributions of the different metabolic systems during the initial stages of a GXT. Thus, the body is able to perform the sub-maximal workloads at a lower oxygen cost with a concomitant reduction in the work performed by the cardiovascular system.