Abstract 1. 1. Changes in blood levels of thyroxine (T 4), triiodothyronine (T 3), free fatty acids (FFA), glucose, pyruvic acid and lactic acid in the harp seal during moult, were studied. 2. 2. Serum levels of both T 4 and T 3 showed significant increase in the moult phase from that in the pre-moult phase. While T 4 level continued to remain high during the early and late post-moult phases, T 3 level dropped in the latter two phases to the same low pre-moult level. 3. 3. The T 3/T 4 ratio was significantly higher during the pre-moult phase than that in all the other phases. It is suggested that the high pre-moult T 3/T 4 ratio marks the initiation of moult. 4. 4. There were no significant changes in the levels of the metabolites studied except that of FFA which was highest in the moult phase indicating the hormonal basis of lipid mobilization.