It is now well established that several hormones are secreted in an episodic or pulsatile fashion (McNatty, Cashmore & Young, 1972; Murray & Corker, 1973). When plasma concentrations of certain hormones are examined, peaks occur at intervals related to different features of the 24-h cycle, circadian rhythms (Retiene, Zimmerman, Schindler, Neuenschwander & Lipscomb, 1968). Circadian and episodic patterns in thyroid function have been claimed by certain workers (Bakke & Lawrence, 1965; Blum, Greenspan & Magnum, 1968) and refuted by others (Schatz & Volpe, 1959; Odell, Wilber & Utiger, 1967). A partial explanation of these conflicting results probably lies in functional differences in the level of the hypothalamo-pituitary-thyroidal-peripheral target tissue axis under investigation by different techniques. Five Jersey bull calves aged between 2 and 80 days were investigated on a total of nine occasions. Five experiments were conducted under natural lighting conditions (daylight from 04.30 to 19.30 h). Calves in the