F.I. KATCH, P.S. FREEDSON, and C.A. JONES. Evaluation of acute cardiorespiratory responses to hydraulic resistance exercise. Med. Sci. Sports Exerc., Vol. 17, No. 1, pp. 168–173, 1985. Accurate evaluation of the acute responses to resistance exercise training depends on the stability of the criterion measures. This is particularly true for maximal effort exercise where continuous “all-out” effort for each repetition is encouraged. The present study evaluated reliability of repetition number (repN), respiratory gas parameters (&OV0312;O2,&OV0312;CO2, &OV0312;E), and heart rate (HR) for shoulder (SE), chest (CE), and leg (LE) exercise performed maximally on a single-unit, 3-station hydraulic resistance exercise machine (Hydra-Fitness, Belton, TX). On 2 separate days, 20 college men completed three 20-s bouts of SE, CE, and LE with a 204 rest between bouts and 5 min between exercise modes. There were no significant differences between bouts or test days for repN, gas measures, or HR. Subjects performed 17, 19, and 21 reps during SE, LE, and CE. &OV0312;O2was 1.7 I-min-1(24.3 ml.kg-1.min-1) for SE, 1.87 I-min-1(25.5 ml.kg-1.min-1) for CE, and 2.1 I-min-1(28.6 ml.kg-1.min-1) for LE. These values, averaged, represented 52.8% of the max &OV0312;O2determined on a continuous cycle ergometer test. The corresponding HR's during hydraulic exercise averaged 84.6% of HR max. Test-retest reliability coefficients ranged from r=.67 to .87 for repN, r=.41 to .83 for gas measures, and r=.72 to .89 for HR. The MET level averaged 7.5 (heavy), and caloric expenditure per minute averaged 35% higher compared with literature values for free weights and 29.4 and 11.5% greater than circuit exercise on Nautilus or Universal Gym equipment, respectively. It is concluded that there are reliable individual differences in repN, respiratory gas parameters, and HR during maximal effort exercise of relatively short duration performed on a multiple-station hydraulic resistance exercise apparatus.