The aim of this study was to localize the areas of the chick brain involved in the development of filial predisposition to follow conspecifics. Expression of the immediate early gene c-fos mRNA was used to map such structures. One-day-old chicks were stimulated (primed) by placing for 90 min into a running wheel in darkness or by exposure to a loud nonspecific sound for 180 min. Brains of a part of these chicks taken 45 min after the beginning of priming were used for c-fos expression study. The remaining chicks were tested 24 hours after priming by simultaneous presentation of a "natural" object (stuffed fowl) and an "artifical" object (rotating red cube). Primed chicks demonstrated significant preference for the natural object as compared to untreated control chicks. In the forebrain of chicks primed both in the running wheel and by acoustic stimulation a significant c-fos expression was found in the medial part of the caudal neostriatum. Priming in the running wheel additionally induced c-fos expression in the lobus parolfactorius, while priming by acoustic stimulation produced high c-fos expression in the archistriatum. Both these areas are known to be involved in filial imprinting. The results suggest that these structures can be also involved in the development of filial predisposition in chicks.