The purpose of this paper is to clarify the main cause of blood resistivity changes during exercise. Hematocrit and blood resistivity were measured during dynamic bicycle exercise, and plasma resistivity was estimated from those measurements. Further, this paper presents an investigation on the effects of hematocrit and plasma on the blood resistivity changes: The mixture function, which is a mixture factor of red blood cells was not correlated with the blood resistivity, but the plasma resistivity was correlated significantly with the blood resistivity. This shows that the plasma resistivity changes strongly and influences the behavior of blood resistivity changes. 