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