The reappearance of circulating catecholamines (CAs) after transfusion with perfluorocarbon blood substitute and the importance of blood for CA conjugation were determined in rats. Arterial plasma concentrations of unconjugated (i.e. free), sulfoconjugated and glucuronoconjugated forms of the CAs dopamine (DA), noradrenaline (NA) and adrenaline (AD) in cannulated, anesthetized, control rats remained stable during the 4.5 h long experiments. Contrarily, in parallel experiments with another group of rats, replacement of blood by the perfluorocarbon blood substitute initially lowered the arterial concentrations of DA glucuronide, free AD and AD glucuronide; subsequently, free DA, NA and AD increased, followed by delayed increases in conjugated CA levels. These changes in CA concentrations preceded a fall in systemic arterial blood pressure (pa) and thus were not the consequences of hypotension.