The relationships between perceived exertion and the physiological variables oxygen uptake (VO2), heart rate (HR) and blood lactate concentration (HLa) were studied in tethered swimming. Seven male and ten female subjects swam at five submaximal intensities. The degree of perceived exertion was measured both on the category scale (RPE) and on the category-ratio scale (CR-10). All five dependent variables (VO2, HR, HLa, RPE and CR-10) were described by the equation R = a+c (S-b)n with the drag (D) in tethered swimming as an independent variable. The relationships between D and VO2, HR and RPE were linear with a level of fitness correlation (r = 0.989-0.999) for both males and females. The HLa exponents were 3.2 for males (r = 1.000) and 3.6 for females (r = 0.991), and CR-10 were 1.5 for males (r = 0.998) and 1.9 for females (r = 1.000), respectively. The growth functions for males and females were very similar. The conclusion is that RPE is effective as a measure of exercise intensity and can be used for exercise prescription in practical swimming.