(1) A simple classification for different types of effort syndrome is outlined. (2) Twenty-five Group 2 (“constitutional”) effort syndromes showed a mean blood lactate rise of 28.9 mgm.%×6.14 s.d. after standard exercise on a bicycle ergometer. The corresponding figure for 35 normal controls was 21.1 mgm.% ×5.89 s.d., which is significantly different. (3) Ten Group 3 (psychogenically produced) effort syndromes did not show a significantly different lactate rise compared with the controls. (4) When worked to exhaustion point on a bicycle ergometer 20 normal controls showed a mean blood lactate rise of 78.0 mgm.% ×8.15 s.d. Twenty E.S. patients gave a mean blood lactate rise of 50.2 mgm. % ×5.74 s.d. (5) Pulse rise and decrement after standard work is highest in Group 2 E.S. and lowest in the normal controls, with Group 3 E.S. occupying an intermediate position.