Comparative studies on protooncogene, c-myc and its heterodimeric partner, max, were carried out using a whole mount in situ hybridization method in Xenopus embryos. Like the expression of Xenopus c-myc, maternal Xmax mRNA was localized in animal hemisphere during early stages (oocyte to midblastula stage), and the expression of both zygotic genes was detected specifically in neuroectoderm and several developing tissues and organs such as optic placodes, CNS, somites, etc. These identical spatio-temporal expression patterns strongly suggest that Xmax and Xmyc gene products interact each other in vivo. Overexpression of Xc-myc gene by microinjection of synthetic RNA resulted in lethality at neurula stage. From the results we suggest that myc/max heterodimers play an important role in early developmental stages especially, after gastrulation when the embryonic induction is initiated.