Concentrations of 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha), the stable hydrolysis product of prostacyclin (PGI2) were determined in venous blood sampled from a forearm vein after dextrose (10% w/v) perfusion and after subsequent distension with physiological saline in seven healthy volunteers. 6-oxo-PGF1 alpha was measured by gas chromatography/negative ion chemical ionisation mass spectrometry. Local release of PGI2 was demonstrated in six volunteers: in three both stimuli were effective and in three only one stimulus was effective. In seven trained athletes studied before and after vigorous exercise baseline plasma concentrations of 6-oxo-PGF1 alpha were less than 1.3-3.2 pg/ml. The concentration increased after exercise in each subject (mean 10.9 pg/ml; range 5.5-18.2 pg/ml). Mild chemical and mechanical stimuli therefore can cause local production of PGI2 from human blood vessels in vivo.