The gene FLT1 produces at least two transcripts from a common transcription start site: full-length Flt1 contains 30 exons encoding a membrane-bound VEGF receptor; soluble Flt1 (sFlt1) shares the first 13 exons but utilizes poly(A) signal sequences within intron 13 to create a transcript that lacks downstream exons. To address the mechanisms that regulate human sFlt1, we mapped the 3` end of sFlt1 mRNA and defined the full extent of its 3` untranslated region (UTR). We identified a 3.2 Kb sFlt1 transcript that is cleaved within an alternatively spliced exon downstream of exon 14 and is predicted to encode a C-terminal variant of sFlt1 with an unusual polyserine tail. sFlt1 mRNA cleavage sites within intron 13 were identified in human placenta and in vascular endothelium by ribonuclease protection assay (RPA). A proximal and two distal mRNA cleavage sites were identified by RPA downstream of consensus polyadenylation signals that create variant transcripts with a 3` UTR ranging from 30 bases to 4 Kb. Northern blot analysis and 3` rapid amplification of cDNA ends (RACE) in placenta confirmed the existence of distal intronic sFlt1 cleavage sites that give rise to a sFlt1 transcript of 7 Kb. The identity of the distal signal sequences were then confirmed by mutagenesis of putative signal elements in a polyadenylation reporter assay. We demonstrate that the heterogeneity of human sFlt1 arises from alternate splicing and from alternative polyadenylation directed by strong intronic poly(A) signal sequences leading to C-terminal variants and to an sFlt1 transcript with a large 3` UTR containing several AU rich elements and poly(U) regions that may regulate mRNA stability.--Thomas, C. P., Andrews, J. I., Liu, K. Z. Intronic polyadenylation signal sequences and alternate splicing generate human soluble Flt1 variants and regulate the abundance of soluble Flt1 in the placenta.