The purpose of this project was to assess the time course of exercise cardiorespiratory changes in response to 17 days of 6° head down tilt bedrest. Seven males (age, 42.3 ±8.7 yr.; height, 181.4 ±6.5 cm; weight, 79.8 ±10.6) completed a continuous exercise test to volitional exhaustion on a supine cycling ergometer prior to bedrest (CON), on days 2 or 3 of bedrest (BR1), 8 or 9 (BR2), and 13 or 14 (BR3), as well as on days 3 or 4 of recovery (R1), and 7 or 8 (R2) to determine the submaximal (150 W) and maximal cardiorespiratory responses to exercise. Submaximal oxygen consumption(VO2) and minute ventilation (VE) did not change from CON during bedrest or recovery. Heart rate (HR: b·min-1) was significantly elevated (p<0.05) from CON (124±4) during BR2 (134±4), BR3(138±4), and R1 (135±2), but had returned to CON levels by R2(128±3). VO2max (L·min-1) was decreased(p<0.05) from CON (3.24±0.20) during BR1 (2.99±0.17; -7.3%), BR2 (3.00±0.17; -7.1%), BR3 (2.92±0.20; -9.0%), and R1(3.02±0.20; -6.6%), but was not different (p > 0.05) than CON by R2(3.13±0.19; -3.3%). HRmax did not change from CON during bedrest or recovery. Initial changes in VO2max (BR1) were significantly correlated with urine balance during bedrest day 1 (r=0.91, p < 0.05). These results suggest that the time course for changes in the cardiorespiratory responses to exercise are not linear and are related to the initial changes in body fluid volumes during 6° head down tilt bedrest.