ATP, ADP and AMP in concentrations at least 1 mumol/l have been found by high pressure liquid chromatography (HPLC) in plasma from peripheral venous blood. Total adenine nucleotide concentrations of about 15-20 mumol/l can be found in some conventional clinical samples of blood using EDTA as an anticoagulant. EDTA prevented adenine nucleotide conversion to inosine in plasma. In order to estimate concentrations in vivo, the contribution derived from the cell breakage inherent even in careful venous blood sampling has been estimated by extrapolation to zero 'haemoglobin' concentration in plasma and minimum values in samples of small volume. Available results appear to be consistent with the release of small amounts of ATP in or near the peripheral circulation at the time of venepuncture. In CSF, ATP, ADP and AMP concentrations were less than 0.05 mumol/l suggesting that membrane activity in the central nervous system is not associated with non-specific leakage. The high Km variant of lymphocyte ecto-5'-nucleotidase was not associated with a significantly higher concentration of its substrate AMP in plasma. However, this enzyme may function on the lymphocyte in the thymus and spleen.