Changes in total thyroxine (T4), free T4 and total tri-iodothyronine (T3) were measured in 13 cats after the intravenous injection of varying doses of thyrotrophin stimulating hormone (TSH) (0–5 U/cat n = 6; 1 U/cat n = 8; 1 U/kg bodyweight, n = 7) or thyrotrophin releasing hormone (TRH) (100 ug/cat, n = 10). All three doses of TSH resulted in a significant (P < 0–05) rise in T4, free T4 and T3 levels, with the mean peak in hormone concentrations occurring six to eight hours after injection. The three doses of TSH all appeared to produce maximal stimulation of thyroid hormone secretion. The mean percentage increase in hormone concentrations at seven hours following the three doses of TSH ranged from 167 to 198 per cent for T4, 240 to 365 per cent for free T4, and 73 to 116 per cent for T3. Following administration of TRH there was also a significant (P < 0–05) rise in T4, and free T4. The mean peak in T4 and free T4 levels occurred at four hours, and mean increases in hormone levels at this time were 92 per cent for T4, and 198 per cent for free T4. The administration of TRH produced little change in T3 levels. TSH administration resulted in a significantly higher (P < 0–05) percentage peak increase in T4, free T4 and T3 levels at all three dosages than did TRH.