The volume of distribution (= VD) of water was measured in Schistosoma mansoni, S. haematobium, S. japonicum, and Hymenolepis diminuta. In the rat tapeworm, H. diminuta, the volume of distribution of 3H-water was positively correlated with wet weight (r = 0.87, P less than 0.001) and this same phenomenon also was demonstrated in S. mansoni (r = 0.90, P less than 0.001). The 5-sec VDwater was constant over a range of glucose (0.01-50 mM) and phenylalanine (0.01-20 mM) concentrations in both male and female schistosomes. The VD of antipyrine, which permeates by virtue of its lipophilic properties, also was shown to correlate with protein content in S. mansoni. Protein content determined in single, isolated schistosomes was correlated with the volume of distribution of water in S. haematobium, S. japonicum, and S. mansoni males and females. Age-related variations in the protein content (of S. mansoni) and volume of distribution of water (in both S. japonicum and S. mansoni) were also defined, and the use of tritiated water content as an indicator of mass in small tissue samples was thus established. Female blood flukes recovered from mice infected for more than 90 days appeared to be characterized by a slight reduction in size with age. Schistosoma mansoni reared in the golden hamster may be slightly smaller than schistosomes of the same strain raised in outbred mice. These results provide a baseline that should be useful for future physiological and immunological studies.