The avian archistriatum, part of which may be homologous with the mammalian amygdala, has been implicated in fear and avoidance behaviour. There is also evidence to suggest its involvement in learning. One-trial passive avoidance learning (PAL) has been used extensively to study memory formation in the chick. Young chicks will peck spontaneously at a small, visually conspicuous bead. If the bead has been coated with an aversive substance, the chicks show a disgust response and learn in a single trial not to peck a similar bead on subsequent presentation. Successful acquisition of this one-trial PAL task involves the formation of a learned association between the bead and a noxious taste, followed by the expression of an avoidance response. In view of the possible involvement of the archistriatum in avoidance and learning behaviour, its role in one-trial PAL was investigated by ablation. Bilateral electrolytic lesions of the entire archistriatum prevented the acquisition of the one-trial PAL task. Neither bilateral lesions of the lateral cerebral area nor sham operation affected learning. The impairment of one-trial PAL caused by archistriatal lesions was not due to effects on the visual or motor components of pecking behaviour. The archistriatum could therefore be directly involved in memory formation. It could also be involved in the organization of avoidance behaviour associated with the task, or it could form part of a circuit linking two other forebrain regions previously implicated in one-trial PAL, the intermediate part of the medial hyperstriatum ventrale and the lobus parolfactorius.