To investigate changes of ryanodine receptors in the sarcoplasmic reticulum (SR) and the nuclear envelope (NE) of rat cardiac myocytes during sepsis induced by cecal ligation and puncture (CLP), myocardial SR and NE were fractionated with density gradient centrifugation and the characteristic of ryanodine receptor was assayed with a method of radioreceptor binding assay. The result showed that Bmax of ryanodine receptors in cardiac SR was increased by 23% during early sepsis (9 h after CLP), but decreased by 38% during late sepsis (18 h after CLP). Bmax of ryanodine receptors in cardiac NE, on the other hand, was increased by 100% and 160% during early and late sepsis respectively. Kd of ryanodine binding to SR and NE remained unchanged during sepsis. These results demonstrated up-regulation of ryanodine receptors in SR occurred during early sepsis and down-regulation of these receptors in SR occurred during late sepsis, while up-regulation of ryanodine receptors in NE occurred during both the early and the late sepsis.