1. Previous evidence has suggested that fluid is extracted from the blood during its passage through the splenic circulation. In order to further investigate this phenomenon, flow probes were placed around the splenic artery and vein of rats. Splenic blood flow and mean arterial blood pressure were measured immediately after surgery, while the rats were still anaesthetized. Five days later, the measurements were repeated first in conscious rats, and then after administering sodium pentobarbitone. Final measurements were made in the conscious animals at day 10 post surgery. 2. Splenic arterial blood flow was lowest at the time of surgery. It had doubled by day 5, and remained stable thereafter at about 8 ml min-1. Splenic venous blood flow also increased after surgery, but less so than the arterial flow, so that the arteriovenous difference had increased to 2.0 +/- 0.4 ml min-1 by day 5. 3. In response to sodium pentobarbitone (20 mg I.V.), there was a transient fall in splenic arterial blood flow from 6.7 +/- 0.9 to 4.9 +/- 0.7 ml min-1. There was no significant change in the arteriovenous difference of blood flow. 4. In conclusion, we have shown that splenic blood flow is considerably higher than previously reported. We have also revealed a significant new path for fluid efflux from the intravascular space; at least 25% of fluid volume flowing into the splen is removed from the circulating blood. 5. Although anaesthesia modulates splenic blood flow, the changes are not as pronounced as those observed in the acute surgically prepared animal.