We previously isolated a p53 cDNA from a Xenopus oocyte library. To determine if p53 has a function in the developmental period, we have studied its expression at the RNA and protein levels during the early development of Xenopus laevis. Two p53 transcripts (3 and 2.2 kb) are expressed from the beginning of Xenopus oogenesis, and the major one (2.2 kb) reaches a level of 7 x 10(5) to 7 x 10(6) transcripts per mature oocyte. After fertilization only the 2.2-kb RNA is detected, but its level decreases and at the neurula stage p53 RNA becomes undetectable. The p53 protein is highly expressed during Xenopus development, in contrast to an undetectable level in Xenopus cells in culture. Most of the p53 protein is synthesized during late oogenesis and a stage VI oocyte contains 7 x 10(11) molecules of p53 protein. This maternal p53 store is maintained at a constant level during Xenopus development, at least until the tadpole stage. This high level of expression is mainly due to stabilization of the p53 protein. Unusually for p53, the protein is strictly located in the cytoplasm of oocytes and this localization might indicate that it is stored in an inactive form at this stage. These data are discussed relative to previous observations made in transgenic mice.