Aptamers (protein binding oligonucleotides) have potential as a new class of targeted therapeutics. For applications requiring chronic systemic administration, aptamers must achieve high-affinity target binding while simultaneously retaining high in vivo stability, tolerability, and ease of chemical synthesis. To this end, we describe a method for generating aptamers composed entirely of 2'-O-methyl nucleotides (mRmY). We present conditions under which 2'-O-methyl transcripts can be generated directly and use these conditions to select a fully 2'-O-methyl aptamer from a library of 3 x 10(15) unique 2'-O-methyl transcripts. This aptamer, ARC245, is 23 nucleotides in length, binds to vascular endothelial growth factor (VEGF) with a Kd of 2 nM, and inhibits VEGF activity in cellular assays. Notably, ARC245 is so stable that degradation cannot be detected after 96 hr in plasma at 37 degrees C or after autoclaving at 125 degrees C. We believe ARC245 has considerable potential as an antiangiogenesis therapeutic.