Immunoreactive beta-endorphin (ir-beta-EP) content in plasma was measured by radioimmunoassay in 101 cardiac patients and 30 normal subjects. The results showed that plasma ir-beta-EP levels in patients of Class II (New York Heart Association Classification), Class III and Class IV were significantly higher than those in the normal group and those in patients of Class I (43.14 +/- 2.80, 54.25 +/- 4.47, 79.28 +/- 8.96 ng/L vs 24.23 +/- 2.13, and 24.98 +/- 3.35 ng/L, respectively, P less than 0.01). Plasma concentration of ir-beta-EP in patients complicated with atrial fibrillation was significantly higher than that in patients without atrial fibrillation (56.27 +/- 4.13 vs 44.66 +/- 3.41 ng/L, P less than 0.05). Ir-beta-EP contents in plasma were correlated positively to the cardiothoracic ratios (r = 0.63, P less than 0.001) and PEP/LVET ratios (r = 0.33, P less than 0.01), and were correlated negatively to the left ventricular ejection fraction and axis shortening (r = -0.41 and r = -0.39, P less than 0.001). These results indicated that plasma ir-beta-EP content may serve as a parameter in evaluating cardiac dysfunction.