Changes in total thyroxine (T4 [TT4]), free T4 (FT4) and total tri-iodothyronine (T3 [TT3]) in serum after the intravenous administration of different doses of thyrotropin (TSH) and thyrotropin-releasing hormone (TRH) were measured in six healthy beagles. Significant (P < 0.05) elevations in serum TT4, FT4 and TT3 were observed at each sampling time (two, four, five, six, seven, eight and 10 hours) after administration of 1, 3 or 5 iu (total dose) TSH and peak mean responses were observed six to eight hours after injection. At six hours after injection the mean TT4, FT4 and TT3 levels were approximately 2.6, 3.9, and 1.5 times basal levels, respectively, and there were no significant differences between the three doses of TSH, Significant (P < 0.05) elevations in serum TT4 and FT4 but not TT3 were observed at each sampling time (two, four, five, six, seven and eight hours) after the administration of TRH. Peak mean responses were observed at four hours after injection at which time TT4 and FT4 levels were approximately 1.7 and 1.9 times basal levels, respectively. No significant differences were observed between the four doses of TRH used (100, 200, 300, and 600 micrograms total dose). Concentrations of TT4, FT4 and TT3 were significantly (P < 0.05) higher following the administration of TSH compared with TRH, and the response to TRH showed greater individual variation.