The disposition in vitro of nitroglycerin (GTN) and its metabolites in erythrocyte suspensions, plasma and blood has been studied using labelled (3H) and unlabelled GTN and GTN metabolites separated by HPLC. GTN was rapidly metabolised (t1/2 3 min) to dinitro-metabolites and subsequently to the mononitronitrates in erythrocyte suspensions containing a therapeutic concentration of GTN (0.8-10 ng/ml). The metabolism of GTN and its dinitro-metabolites was concentration dependent. GTN and its dinitro-metabolites were plasma protein bound (11-60%). They had an apparent erythrocyte-plasma partition coefficient in the range 0.6 to 0.9. The metabolism of GTN by erythrocytes was partially inhibited by the presence of the dinitro-metabolites of GTN. The metabolism of GTN in blood samples collected from patients could be stopped by adding the collected blood to chilled tubes containing the enzyme inhibitor iodoacetamide.