Unlike N-desmethyl-1-alpha-acetylmethadol (NLAAM), a single injection of methadone (METH), near domestic chicken embryos early during development, cannot induce and sustain opioid-type dependence in the older embryo (i.e., days 14-17 of development). Injection of [3H]-METH near the 14-day-old embryo, followed by differential extraction, indicated that significant quantities of unmetabolized METH gained entrance to the brain, peaking at about 1 hr and declining with a half-life of about 2.8 hr. Thus, it is probably not practical to use a single injection of this shorter-acting opioid for studying biobehavioral effects of sustained dependence and withdrawal during development in this species. Chronic infusion of METH for 7 days via an externalized Alza osmotic mini-pump resulted in significant, dose-dependent brain concentrations of [3H]-METH on day 14. Even though the opioid antagonist naloxone (Nx) was unable to induce withdrawal, manifest as a significant increase in embryonic motility above that of controls, it partially reversed the depressed motility caused by the chronic infusion of [3H]-METH. Since 7-day-old embryos exposed to NLAAM, at doses which can be demonstrated to produce dependence by precipitating withdrawal on day 17 of development, were also unable to express withdrawal on day 14, it is possible that either 14-day-old chicken embryos cannot yet fully respond with an adaptive process (i.e., dependence) or its expression (i.e., withdrawal).