Abstract. This study was undertaken to determine the effect of opioid peptides and naloxone on the secretion of thyrotrophin (TSH), alpha subunit (α subunit) and beta thyrotrophin (TSH-β) from rat pituitary dispersed cells in primary culture. Naloxone (NAL) 10−5m was found to increase basal TSH, α subunit and TSH-β secretion. This effect of NAL was not blocked by human β-endorphin (βh-End) 10−7 m. Concurrent treatment with triiodothyronine (T3) 10−7 m significantly decreased NAL stimulated secretion of TSH and its subunits. Thyrotrophin releasing hormone (TRH) stimulation of secretion of TSH and its subunits was not further augmented by NAL. In contrast, 10−7 m of βh-End, methionine-enkephaline (Met-Enk) and D-ala2-met-enkephalinamide (DALA) had no effect on secretion of TSH and subunits. A time course study confirmed no change in TSH secretion following pre-treatment with βh-End at 4, 10, 24 and 48 h. These findings go against a direct action of βh-End, Met-Enk and DALA on TSH secretion. The response of TSH and its subunits to NAL and the lack of interaction with βh-End might be explained by the existence of different types of opiate receptors. Counteraction of this effect by T3 suggests other possible mechanisms.