The effects of thyroxine (subcutaneous injection) and methimazolum (intubation feeding) on thermosensitive neurons (TSN) in the preoptic/anterior hypothalamic (PO/AH) area in rats were observed. The ratio of warm-sensitive neurons (WSN) to cold-sensitive neurons (CSN) was 1.86:1 and thermo-insensitive neurons (TIN) to thermosensitive neurons (TSN) 1:1.43 in the control group. The ratio of WSN to CSN decreased to 1.20:1 and the ratio of TIN to 1:2.36 in the thyroxine group, while the former increased to 2.40:1 and the latter decreased to 1:1.29 in the methimazolum group. There was significant difference among three groups, (X2 = 9.64, P less than 0.05). Thirty-two percent of neurons (11/34) showed higher firing rates (greater than 15Hz) in the control group, only 8% (3/37) in the thyroxine group and 9% (3/32) in the methimazolum group. In addition, the tolerance of TSN to warming stimulation was obviously lower in both pathological groups, as compared with the control group. The results suggest that a rise or fall of body temperature in rats with subcutaneous injecting thyroxine and intubation feeding methimazolum may be related to the changes of proportion and excitability of PO/AH neurons, and that the disturbance of thyroxine synthesis, release and metabolism may interfere temperature regulation at the level of hypothalamus.