We measured muscle sympathetic nerve activity (MSA) to clarify the mechanisms of the blood pressure rise during cold pressor test (CPT), simultaneously with impedance cardiography and blood pressure wave measurement in 10 healthy subjects. MSA remained unchanged during the initial period of 0—30s of the CPT and increased remarkably during the later period of 30—90s of the CPT, while cardiac output exhibited a slight increase during the initial period but not during the later period. Mean blood pressure increased significantly throughout the entire period of CPT and reached the maximal level during 90—120 s of the CPT. The mean blood pressure and total peripheral resistance during the CPT showed a linear correlation with MSA. In conclusion, an increase of cardiac output elevates blood pressure at the initial period of the CPT with little increase in MSA, while an increasc of MSA plays an essential role to elevate the blood pressure at the later period of the CPT.