Human presenilin (ps)-1 and -2 genes have recently been shown to be involved in genesis of early-onset familial Alzheimer's disease. By probing with human (H-) ps-1 cDNA, we isolated two types of cDNA clones, named X-ps-alpha and -beta, from a Xenopus brain cDNA library. The encoded proteins, X-PS-alpha and -beta, may correspond to H-PS-1 and -2 with 89.4 and 85.9% similarity, respectively. The strongest expression of these genes was observed in ovaries and in the early stages of oogenesis, although weak or moderate expression was detected ubiquitously for both X-ps-alpha and -beta genes in multiple tissues. Upon oocyte maturation, the X-ps-beta mRNA level was constant even after fertilization until the midblastula transition. Zygotic expression of these genes became evident only at the tailbud stage. We propose that presenilins may function in preventing cells from undergoing apoptotic degeneration particularly prior to embryonic development and in developmentally matured tissues.