A canine and feline pituitary-thyroid function test based on thyrotropin-releasing hormone (TRH) stimulation of endogenous thyrotropin is described. Serum thyroxine is measured before and after stimulation with TRH. A positive response to TRH indicated a functionally intact pituitary-thyroid axis. At TRH doses of 0.002 to 10.0 mg/kg of body weight, dose response of serum thyroxine to TRH stimulation was determined. Increasing the dose of TRH increased the duration, but not the magnitude, of thyroxine stimulation. At TRH doses greater than 0.1 mg/kg, drug side effects were salivation, defecation, urination, vomition, miosis, tachycardia, and tachypnea. A useful procedure for pituitary-thyroid function testing was serum thyroxine measurement before and 6 hours after TRH (0.1 mg/kg) stimulation.