Measurements of blood and plasma volumes using 131I-albumin, and of red cell volume using 51Cr in goats and sheep infected with Trypanosoma vivax for 1 to 2 months (at which time the anaemia was severe) showed statistically significant increases in blood volume (29 per cent and 57 per cent) and plasma volume (44 per cent and 59 per cent), and decreases in red cell volume per kilogram body weight (49 per cent and 50 per cent) in goats and sheep respectively. Total serum proteins and gamma globulins increased, while serum albumin decreased, in T. vivax infected sheep and goats. These findings indicate that the anaemia manifested, with mean packed cell volume decreased by 60 per cent and 47 per cent in goats and sheep respectively at the time of red cell volume measurements, is attributable partly to haemodilution and partly to an actual decrease in total volume oc circulating red blood cells.