35S-sulphate has been injected into the body cavity of yearling Rainbow trout (Salmo gairdneri) and the time sequence of its excretion, and of its uptake and binding by branchial cartilage, determined by scintillation counting. The data are shown to conform well with the time sequence of movement of 35S-sulphate into the cells and matrix of the cartilage, as visualized by autoradiography. Peak content of both unbound and bound sulphate is reached within the cartilage at about 30 hours after injection, in both control and thryoxine-treated fish, but the uptake in the latter is significantly higher. In conformity with this, the rate of excretion of 35S-sulphate is lower in the hormone-treated animals. The results confirm and extend earlier work in showing that thyroxine, by enhancing sulphate uptake, could exert a growth-promoting influence upon cartilage, but it is pointed out that this can be only one component of a complex structural and hormonal situation.