A comparison of the effect of PTU-induced hypothyroidism on the contents of immunoactive met-enkephalin and beta-endorphin in the pituitaries from rats aged 2 1/2, 12, and 18 months was made. In all ages, there was significant reduction of IR-met-enkephalin content in the anterior lobe and IR-beta-endorphin content in the neuro-intermediate lobe after PTU treatment. There was a significant age-related decrease in IR-beta-endorphin content in the anterior lobe. Rats of all three ages responded to PTU treatment with an increase in serum TSH level and a drastic reduction in serum T3 and T4 levels. The results indicate that there was no age-related difference in the change of pituitary opioid peptide contents in response to hypothyroidism.