Although a great deal is known concerning the pathophysiology of sepsis, it is not clear whether circulating blood volume (CBV) is altered under such conditions. To study this, rats were subjected to sepsis by cecal ligation and puncture (CLP). Immediately after CLP or sham operation, the animals received 3 ml/100 g body weight normal saline subcutaneously. CBV was determined by using in vivo indocyanine green (ICG) clearance at 2, 5, 10, or 20 hr after CLP or sham operation. This technique does not require any blood sampling. Serum glutamic pyruvic transaminase (SGPT) and glutamic oxaloacetic transaminase (SGOT) were assayed enzymatically as indicators of hepatocyte damage. Hepatic microcirculation was assessed at a selected time point (10 hr post-CLP) by using laser Doppler flowmetry and colloidal carbon infusion techniques. The results indicate that CBV, as determined by ICG clearance, remained unchanged up to 10 hr following the onset of sepsis (i.e., early sepsis) but decreased significantly at 20 hr after CLP (late sepsis). However, systemic hematocrit increased significantly at 5, 10, and 20 hr after CLP, indicating that plasma volume decreased at those time points. This suggests that there may be limitations in accurately assessing CBV at 5 and 10 hr after CLP, i.e., during the hyperdynamic circulatory state of sepsis, using the ICG clearance method. Moreover, SGPT and SGOT levels increased significantly at 10 hr, and the levels increased further at 20 hr post-CLP. In contrast, microvascular blood flow and carbon-perfused areas in the liver were significantly increased at 10 hr post-CLP. Thus, the hepatocyte damage, which occurs in early sepsis at 10 hr after CLP (as demonstrated by elevated SGPT and SGOT levels), is not due to any reduction in hepatic microcirculation, but may be due to elevated inflammatory cytokines.