Abstract: Described herein are methods for improved partitioning between high and low affinity nucleic acid ligands identified through the SELEX method, termed solution SELEX. The solution SELEX method achieves partitioning between high and low affinity nucleic acid-target complexes through a number of methods, including (1) primer extension inhibition which results in differentiable cDNA products. Primer extension inhibition is achieved with the use of nucleic acid polymerases, including DNA or RNA polymerases, reverse transcriptase, and Q.beta.-replicase; (2) exonuclease hydrolysis inhibition which results in only the highest affinity ligands amplifying during PCR. This is achieved with the use of any 3'.fwdarw.5' double-stranded exonuclease; (3) linear to circle formation to generate molecules amplifiable during PCR; or (4) PCR amplification of single-stranded nucleic acids.