1. The total amounts of urea and trimethylamine oxide (TMAO) increase during the development of the oviparous elasmobranch Raja binoculata. These increases are not explained by increases in the concentrations, which, when measured on the basis of water content, either decline (TMAO) or remain relatively constant (urea).2. The increases in urea and TMAO are correlated with an increase in the total volume of the embryonic system during development.3. The increase in the total volume of the system is due mainly to an increase in its water content. This, in turn, is correlated with a higher percentage of water in the developing embryo than in its yolk.4. The increase in the total amount of TMAO during development would strongly suggest that the embryo is capable of producing this compound, particularly since the embryo is apparently a closed system with regard to organic subtrates.5. There was a definite, though variable, loss of urea from the embryonic system but not from the undeveloped egg. There was, however, no detectable loss of TMAO. The amount of urea lost per day and the degree of variability increased during development. The undeveloped egg is apparently able to retain urea until such time as the embryo is capable of producing this compound.6. The maintenance of relatively constant urea and TMAO concentrations implies that regulation occurs throughout development, and that these organic solutes play a similar osmotic role in the embryonic system as in the adult.7. The ability to retain relatively constant urea levels by even the earliest stages of this oviparous form would seem to make less tenable the suggestion of other authors that the need to retain urea was a selective pressure leading the elasmobranchs toward intrauterine development.