Five subjects performed 6 min of cycling exercise at a constant resistance (2 kg) and three pedalling frequencies (30, 50 and 75 rev.min-1). The resulting exercise intensities required 32, 45, 68% of their average Vo2max. Electromyographic tracings (EMG) from the left quadriceps and magnitude estimates of overall effort sensations were made continuously. There was no difference in the area of the smooth rectified EMG resulting from frequency or number of contractions. Slopes of the effort-time functions were not significantly different between 30 and 50 rev.min-1 and thus raised the issue of discriminability between cycling exercise intensities. The difference in slope between 50 and 75 rev.min-1 was significant (p less than 0.01). Oxygen uptake averaged for all subjects reached steady state between about 2-3 min of exercise but magnitude estimates of effort increased continuously during exercise. Thus, no correspondence was found between effort and Vo2 when the peripheral signal to the effort sense was held constant. It is concluded that the second input to the effort sense does not arise from cardiovascular and respiratory responses to oxygen demand from the periphery.