The blood volume of T. vulpecula was measured and the influence of the spleen on the circulating blood volume investigated. The circulating blood volume of "normal" restrained animals was 57.4 � 3.19 ml, the plasma volume being 31.2 � 1.93 ml, and the red blood cell volume 26.2 � 2.08 ml per kilogram body weight. These values tended to be lower than those of eutherian mammals and it is suggested that this might be associated with a possible lower metabolic rate. The spleen was found to have a significant function as a blood reservoir. Measurement of volume of circulating red blood cells after injections of adrenaline (to cause splenic emptying) and chlorpromazine (to achieve maximum filling of the spleen) showed that the splenic reserve of erythrocytes was approximately 11.0 ml/kg body weight.