Immunoreactive endothelin-1 (ET-1) levels were measured in the pericardial fluid (PF), plasma (PL), and atrial and ventricular tissues of 16 cardiac surgical patients. ET-1 was higher in the PF (73 +/- 11 pg/ml) than in the PL/3.33 +/- 0.51 pg/ml; p < 0.05/. PF/PL ratio ranged from 8 to as high as 200. An inverse relationship was found between the NYHA stage and ET-1 concentrations in PF, whereas ET-1 in PL did not have any relation to the NYHA stage of the disease. In HPLC analysis, the total immunoreactive ET-1 in PF from five different patients co-eluted with the human ET-1 standard. The results indicate that ET-1 concentrations in human PF can be several-fold those in the PL. The source and function of the ET-1 in PF requires further investigation.