The nymphalid butterfly Araschnia burejana and the papilionid butterfly Papilio xuthus exhibit seasonal diphenism comprising spring-morphs that develop from diapause pupae and summer-morphs that develop from non-diapause pupae. The development of seasonal morphs in A. burejana is regulated by the timing of secretion of ecdysteroids for adult development, whereas that in P. xuthus is regulated by the secretion of summer-morph-producing hormone, which is present in the brains and is under control of the photoperiod. We investigated whether a cerebral factor derived from brains plays a significant role in the regulation of seasonal morph development in A. burejana using surgical operations. Pairs of chilled diapause pupae that had been chilled for more than 3 months at 4°C were joined surgically to each other and then developed into spring-morph or spring-like-morph adults. Chilled diapause pupae that were joined with 1-day-old non-diapause pupae developed into summer-morph or summer-like-morph adults. When the brains of non-diapause pupae were removed surgically 6-8 hr after pupation with and without injection of 20-hydroxyecdysone, a large portion of them developed into spring-morph or spring-like-morph adults, respectively. Furthermore, 90% of non-diapause pupae developed into spring-morph or spring-like-morph adults when the neck was ligated within 5 min after pupation. These results indicated that a cerebral factor showing summer-morph-producing hormone activity, which is secreted from the brain in the early pupal stage, in addition to 20-hydroxyecdysone for adult development, play a significant role in the determination of summer-morph development in non-diapause pupae of A. burejana.