The equilibrating process of catecholamines (CAs) between plasma and red blood cells (RBC) was studied by measuring their erythrocyte/plasma concentration gradient (E/P ratio); ratio of E/P > 1 for a given amine was considered the consequence of its accumulation in or on RBC. We studied in vitro human blood obtained from 9 polycythemic patients from whom blood was drawn in control condition and in response to loading with exogenous CAs. Preliminary study showed a lack of difference in results obtained in these patients from those in 9 healthy volunteers and confirmed that RBC accumulate dopamine (DA) and epinephrine (EPI) but not norepinephrine (NE). When a moderate amount of exogenous CAs was added, plasma and RBC concentrations were increased, with a delay between the two compartments indicated by a decrease in the E/P ratio of DA and EPI. When a large amount of exogenous CAs was added, their RBC concentrations were in direct proportion to their plasma concentrations. Thus, the kinetics of CA equilibration between plasma and RBC appears to be dependent on their chemical structure (DA is more easily accumulated than NE) and their plasma concentrations. Physiologically, the E/P ratio of DA was significantly greater than 1, suggesting that RBC maintained their capacity to accumulate DA even when DA plasma concentration was very high. Additional studies demonstrated that accumulation of CAs in or on RBC is a reversible process, inhibited by cold temperature and increased when blood pO2 is drastically reduced.(ABSTRACT TRUNCATED AT 250 WORDS)