Hemoglobin (Hb) and lactate (La) concentrations were measured in small (150 microliters) blood samples collected with micropipettes from the inflow and outflow vessels of the rete mirabile of the eel swimbladder. Hemoglobin was used as a marker of the intravascular space. 1. Hemoglobin concentrations suggest that there was no significant water movement between the arterial and venous capillaries in the rete, but a significant 6% water efflux from the vascular space into the swimbladder epithelium. No significant differences in osmolality were observed between the sites of measurement. 2. Of the lactate present in the blood entering the venous capillaries of the rete, 30% derived from release by the swimbladder epithelium; 38% of the lactate entering the venous capillaries diffused back in the rete tissue into its arterial capillaries. 3. Theoretical models suggest that any water movement in the swimbladder, leading to blood flow mismatch in the rete counter-current system, reduces its efficiency to concentrate inert gases, whereas lactate back-diffusion enhances this efficiency.