Adult female Sprague-Dawley rats were prepared with chronic cortical and muscle electrodes and i.v. cannulas, made tolerant to and physically dependent on morphine, and trained to level press for i.v. morphine self-injections to maintain dependence. Methadone or l-alpha-acetylmethadol (LAAM) was then substituted for morphine in some of these rats. During self-maintained dependence on either morphine or methadone, head shakes appeared and increased in frequency before lever pressing for self-injections. In contrast, there were fewer head shakes during LAAM dependence, which were evenly distributed over the entire duration of the interinjection interval. These findings suggest a relationship between head-shake distributions, drug-seeking behavior and the pharmacodynamics of these three narcotics.