Effects of three consecutive days injections of thyroxine (T4) of different doses (1, 2 and 4 micrograms/g of body weight) on different parts of the brain of Singi fish were studied. T4 enhanced the cranio-somatic index and the weight of different substructures of brain, viz., cerebrum, cerebellum midbrain and medulla oblongata, of Singi fish at 25 degrees C. Compared to the control, the protein content of cerebrum increased with all doses of T4 (1, 2 and 4 micrograms/g) and the RNA content of this region increased only with higher doses (2 and 4 micrograms/g). Cerebellum appeared to be relatively less responsive than cerebrum; T4 at the dose of 4 micrograms/g was only effective in increasing the protein content of the cerebellum, although the RNA content of this region was enhanced with both 2 and 4 micrograms of T4 per g. In midbrain and medulla oblongata, the protein content was elevated only with higher doses (2 and 4 micrograms/g) and RNA content was enhanced with all doses of T4 used. T4 injections did not cause any alteration of the DNA content of cerebrum and cerebellum, but in midbrain this cellular constituent increased in amount with all doses of T4 and in medulla oblongata it was increased with the dose of 2 or 4 micrograms of T4 per g body weight.