A sequential study of the effect of thyroxine (T4) on the protein composition of the colloid in the rat thyroid gland has been carried out in 24 rats. Daily ip injections of 20 mug thyroxine were given and the colloid was analyzed after 1, 2, 3 and 4 days and 1, 2 and 3 weeks. The protein constituents were separated by microgel electrophoresis and measured by microdensitometry. The incorporation of [3H]leucine into the thyroglobulin fraction and into the albumin-like protein in the homogenate of the gland was analyzed after T4 suppression for 2 days. The 2 h thyroidal iodine uptake was analyzed during the first week of T4 administration. During the second and the third day of T4 administration, the amount of 27S and larger complexes of thyroglobulin was reduced by 30-40 per cent and the amount of the albumin-like protein was significantly increased and present in almost all samples. Preliminary data indicated that during the same period the relative amount of [3H]leucine activity was increased in the albumin-like protein fraction compared to the thyroglobulin fraction in the thyroid. It is concluded that the exocytotic vesicles contain the albumin-like protein. The results further indicate that the administration of T4 temporarily changes the relative rates of formation of thyroglobulin and of the albumin-like protein in the follicle.