This paper reports the effects of chronic postnatal methadone administration on basal hormone levels and on the endocrine response to methadone. Chronic postnatal methadone administration delayed normal developmental increases in serum T4 (decrease on day 5), corticosterone (CS) (days 15 and 20) and growth hormone (GH) (day 25). In addition, marked tolerance to methadone effects on GH, CS, thyroid-stimulating hormone (TSH) and prolactin (PRL) secretion developed during chronic postnatal administration, and effects on PRL and TSH secretion persisted after drug withdrawal.