T h e composition of elasmobranch plasma differs strikingly from that of mammals. Urea concentrations of extracellular fluid (ECF) are greatly elevated as compared to mammals. In addition, extracellular concentrations of Na+ and C1-are substantially higher while extracellular concentrations of total COz( HCO-3 and H2C0,) are substantially lower (Smith, '29). No systematic survey has been made of the electrolyte composition of intracellular water in the elasmobranch. As a result, the mechanisms responsible for the regulation of intracellular electrolyte composition have not been specifically considered. It is the purpose of this paper to report the water content, Na+, K+, H+, and C1-concentrations of erythrocyte, muscle and brain in a typical elasmobranch, S . acanthias.
METHODS AND MATERIALS
A total of ten fish were studied. Each animal was studied within 2 to 3 hours of capture and maintained in a sea water pen during the 4 to 5 hours necessary for study. At the beginning of each study, 10 ml of blood was obtained anaerobically from a caudal vessel and 0.25 gm of 5,5dimethyl-2,4-oxazolidinedione (DMO ) dissolved in 10 ml of sea water was injected so that intracellular pH could be determined. This sample of blood was centrifuged for 20 minutes and the plasma and red cells separated. The quantity of DMOlike substance was determined in this control sample of plasma and of erythro-