Gene expression is extensively regulated at the levels of mRNA stability, localization and translation. However, decoding functional RNA-regulatory features remains a limitation to understanding post-transcriptional regulation in vivo. Here, we developed RNA-element selection assay (RESA), a method that selects RNA elements on the basis of their activity in vivo and uses high-throughput sequencing to provide a quantitative measurement of their regulatory functions at near-nucleotide resolution. We implemented RESA to identify sequence elements modulating mRNA stability during zebrafish embryogenesis. RESA provides a sensitive and quantitative measure of microRNA activity in vivo and also identifies novel regulatory sequences. To uncover specific sequence requirements within regulatory elements, we developed a bisulfite-mediated nucleotide-conversion strategy for large-scale mutational analysis (RESA-bisulfite). Finally, we used the versatile RESA platform to map candidate protein-RNA interactions in vivo (RESA-CLIP).