To estimate catecholamine (CA) release during hemodialysis (HD), plasma-free and conjugated CAs and their dialyzer clearance rates were measured in 10 HD patients (age; 49.8±15.2 years, duration of HD; 5.8±5.0 years). Although free dopamine (f-DA) and all conjugated CAs decreased to about one half of the pre-HD levels at the end of HD, no significant change was seen in free norepinephrine (f-NE) and epinephrine (f-E) during HD. For every CA, the clearance rate was the highest in the sulfate and the lowest in the glucuronide form, and NE was the highest in every form. In the comparison between the measured CA and calculated CA using the clearance rate and the pre-HD level, the measured values of f-NE and f-E were significantly higher than the calculated values, unlike the results for f-DA and conjugated CAs. The difference in f-NE between the measured and the calculated values correlated negatively with the change in mean blood pressure (ΔMBP), and ΔMBP was also correlated with the ultrafiltration volume. From these data, it was suggested that f-NE was released by the decrease of MBP due to the increase of ultra-filtration volume during HD.