In mouse oocytes, tissue-type plasminogen activator (tPA) mRNA is under translational control. The newly transcribed mRNA undergoes deadenylation and translational silencing in growing oocytes, while readenylation and translation occur during meiotic maturation. To localize regulatory elements controlling tPA mRNA expression, we identified regions of the endogenous transcript protected from hybridization with injected antisense oligodeoxynucleotides. Most of the targeted sequences in either the 5' untranslated region (5'UTR), coding region, or 3'UTR were accessible to hybridization, as revealed by inhibition of tPA synthesis and by RNase protection. Two protected regions were identified in the 3'UTR of tPA mRNA in primary oocytes: the adenylation control element (ACE) and the AAUAAA polyadenylation signal. These sequences were previously shown to be involved in the translational control of injected reporter transcripts. During the first hour of meiotic maturation, part of the ACE and the AAUAAA hexanucleotide became accessible to hybridization, suggesting a partial unmasking of the 3'UTR of this mRNA before it becomes translationally competent. Our results demonstrate that in vivo antisense oligodeoxynucleotide mapping can reveal the dynamics of regulatory features of a native mRNA in the context of the intact cell. They suggest that specific regions in the 3'UTR of tPA mRNA function as cis-acting masking determinants involved in the silencing of tPA mRNA in primary oocytes.