The cytochemical method of HOWELL and WHITFIELD (1972) and the revised cytochemical medium of KEMPEN et al. (1978) has been used to investigate the localization of adenylate-cyclase activity in 18, 19 and 20 day old first embryonic mandibular molars. Membrane associated precipitate granules were observed in preameloblasts and odontoblasts localized in the area where preodontoblasts become post-mitotic. Heat inactived teeth and the samples incubated in media without substrate were negative for membrane bound lead deposits. A correlation may exist between high adenylate-cyclase activity and the terminal differentiation of odontoblasts.