The role of the pineal gland in the regulation of thyroid-stimulating hormone (TSH) secretion was studied in postpuberal male rats using 12:12 h light:dark cycles. Pinealectomy, performed 3–4 days before decapitation, had no effect on the basal or thyrotropin-releasing hormone stimulated TSH secretion, but significantly decreased the cold-induced TSH response both at noon and at midnight. Neither hypothalamic thyrotropin-releasing hormone concentrations nor serum thyroid hormone levels were affected by pinealectomy. Melatonin injected intravenously (1 and 10 mg/kg) or intracerebroventricularly (5 and 50 µg/rat) had no effect on the basal or cold-stimulated TSH levels, neither was the thyrotropin-releasing hormone induced (50 and 125 ng/rat i.p.) TSH response modified by melatonin (1 mg/kg i.v.). Arginine vasotocin (1 and 100 ng/rat) given intracerebroventricularly had no effect on the basal or cold-stimulated TSH secretion. These results suggest that the pineal has no influence on the basal activity of the hypothalamus-pituitary-thyroid system, but it seems that the cold-induced TSH response depends to a considerable degree on the intact pineal gla