Recent reports on the in-vivo and in-vitro effects of the β-adrenergic blocking agent propranolol (Inderal, I.C.I.) (Burke, 1969; Franco, 1970; Melander, 1971; Biran & Tal, 1972) have arrived at contradictory conclusions regarding its effect on thyroid activity. The present paper is devoted to clarification of this problem in vivo by investigating: (a) the influence of propranolol on thyroid secretion, (b) the direct or indirect effect of propranolol on the thyroid gland, and (c) involvement of the hypothalamus—pituitary system. Intact rats. Propranolol (1–5 mg/kg) was administered daily i.p. in 0·1 ml propylene glycol to 32 male 21-day-old rats of the Hebrew University Sabra strain, weighing 42 ± 2 g. The rats were divided into four equal groups (nos. 1–4) and received standard Purina chow and water ad libitum. The cages were kept at 23 ± 1 °C and exposed to artificial light for 12 h daily. On the 42nd day of life the