Chicken, rat and human U1A RNAs in solution, were examined for secondary structure, using several methods including hydrolysis by various nucleases, hybridization to DNA oligomers and analysis of fragment interactions. The experimental results showed that the three U1A RNAs have the same structure, stable over a wide range of pH and ionic conditions. They allowed the selection of one out of several possible models constructed from the data of primary structure. This model is characterized by 4 hairpins and two single-stranded regions, the two hairpins from the 3' part of the molecule bearing very stable stems. In addition, the experimental results showed that in contrast to the 5' half of the molecule, the 3' half has a compact conformation probably stabilized by tertiary interactions. The 5' end of U1A RNA is accessible and free of base-pairing so that it might base-pair with regions of other RNA molecules, for instance, with the extremities of introns as has been recently proposed in a model of splicing.