The work reported here was an attempt to determine the concentrations of thyroxine (T4) and tri-iodothyronine (T3) in the blood of the common toad (Bufo bufo). Few such results are available for cold-blooded animals. The toads were pithed, pinned out supine, and the heart and its associated blood vessels were carefully exposed. Blood was obtained by puncturing the truncus arteriosus and applying suction, the blood collection apparatus having been previously heparinized. The yield was improved by tying off the aortic arches close to the heart and cannulating each aorta in turn with a flow of saline (Ringer's solution with 5% albumin added). The dilution thus caused was estimated from the haematocrits of the blood before saline was administered, and of the blood-saline mixture at the end of the collection. After centrifugation, the supernatant was stored at − 10° until required. The supernatants from several toads were combined, adjusted