1. After an 80 min intravenous infusion of 1-[ 3 H]noradrenaline ([ 3 H]NA) in 19 subjects, the proportion of alumina-extractable 3 H due to [ 3 H]NA was 86 ± 2% in arterial and 78 ± 2% in venous plasma. [ 3 H]Dihydroxy-phenylethyleneglycol ([ 3 H]DHPG) accounted for 8 ± 1% in arterial and 13 ± 1% in venous plasma. [ 3 H]Dihydroxymandelic acid ([ 3 H]DOMA) was not detected. 2. No time-dependent change in the proportion of alumina-extractable 3 H due to [ 3 H]NA was seen in either arterial or venous plasma over this period. In venous plasma the proportion of alumina-extractable 3 H due to [ 3 H]DHPG increased slightly between 30 and 80 min, from 10 ± 1% to 13 ± 1%. 3. The results were unchanged with chronic β-blockade and with either 1-[2,5,6- 3 H]NA or 1-[7,8- 3 H]NA. 4. After [ 3 H]NA infusion in man the radioactivity recovered after alumina extraction is predominantly [ 3 H]NA and thus can be used directly to determine [ 3 H]NA kinetics without the need for metabolite separation.