An understanding of the conditions influencing protein binding of catecholamines (CAs) is important in studying their metabolic effects. Unfortunately, reports on plasma protein binding of CAs are scarce, conflicting and mainly performed in vitro. The aim of our in vivo and in vitro studies was to investigate binding and clearance of radioactive adrenaline (epinephrine) ((3)H-A), noradrenaline (norepinephrine) ((3)H-NA) and their metabolites in sheep blood. The time course of the radioactivity in the blood after intravenous injection of (3)H-A and (3)H-NA (3.7 MBq each) in 4 sheep (2 of each sex; total of 8 administrations) was determined. Blood samples were taken from the jugular vein. The highest radioactivity was observed in the first sample (5 min) following injection. Radioactivity showed a biphasic disappearance. An initial stage, in which radioactivity decreased rapidly (within 1 h) after the injection, was followed by a slow stage, lasting for up to 1 month, until background levels were reached. In vitro results indicated that NA and A were present not only in plasma (70%) but also in the erythrocytes (30%; mainly bound to haemoglobin). Sephadex G-25 gel filtration revealed that from the plasma fraction about 15% was strongly bound to proteins (mainly albumin). These results demonstrate that previous experiments in this field have overestimated the percentage of CAs bound to plasma proteins, because binding to haemoglobin was previously not known. In the future, efforts should be made to characterize the adduct products of CAs and establish an assay to determine them in vivo. If this could be achieved, it would yield a valuable tool for measuring the stress experienced for a longer period.