The possible role of urea and trimethylamine oxide (TMAO) in providing positive buoyancy has been examined for elasmobranch fishes. TMAO has a considerably lower density than an equimolar solution of urea, and solutions of both TMAO and urea are considerably less dense than equimolar solutions of most other body fluid solutes. The body fluid composition of three elasmobranchs, the whiskery shark Furgaleus ventralis, the black whaler shark Carcharhinus obscurus and the shovelnosed ray Aptychotremata vincentiana, is typical for marine elasmobranchs, with plasma concentrations of about 260 mmol l-1 Na+, 250 mmol l-1 Cl-, 340 mmol l-1 urea and 70 mmol l-1 trimethylamine oxide. A plasma density of 1.015 was calculated for the whaler shark (from the concentrations, relative molecular masses and absolute molal volumes of plasma solutes), which would contribute a positive lift of 8.45 g l-1. There is a large positive contribution to buoyancy by urea (3.7 g l-1), trimethylamine oxide (1.8 g l-1) and Cl- (4.0 g l-1), whereas slight negative buoyancy is conferred by Na+ (-0.8 g l-1). Divalent cations (Ca2+, Mg2+) contribute minimal negative buoyancy (about -0.1 g l-1 each) despite their rather negative partial molal volumes, because of their low concentrations. Muscle fluids contain about 40 mmol l-1 Cl-, 365 mmol l-1 urea, 160 mmol l-1 trimethylamine oxide, 16 mmol l-1 betaine and 69 mmol l-1 sarcosine. The organic solutes contribute about 12.1 g l-1 lift. Although urea and TMAO act as balancing osmolytes, and TMAO as a counteracting solute, a positive buoyancy role must be considered as a further adaptive function of urea and TMAO accumulation in chondrichthyean fishes.