Visna virus RNA consists of a high molecular weight and low molecular weight species. The size, subunit composition, complexity, and secondary structure of high molecular weight RNA have been studied and compared with Rous sarcoma virus RNA. Visna high molecular weight RNA cosediments in 0.1 M sodium chloride wiih the 70S UNA of Rous sarcoma virus consistent with a molecular weight of 10-12 × 10 daltons. On dissociation with heat, subunit structures of 2.8 × 106 daltons are released. Both the size and heterogeneity of the subunit RNA are identical with the subunit RNA of transforming Schmidt-Ruppin Rous sarcoma virus. Comparison of the complexity of visna RNA with poliovirus RNA indicate unique nucleotide sequences of 7-10 × 106 daltons, in accord with the physical data. The evidence also indicates that there is lack of significant reiteration of nucleotide sequences in the visna RNA. Major differences in the secondary structure of visna and Rous sarcoma virus RNA were observed. Both RNAs possess an exceptional degree of secondary structure when assayed by chromatography on cellulose at various temperatures. Reduction of ionic srength markedly decreases the secondary structure of visna RNA relative to Rous sarcoma virus RNA and leads to anomalous migration in polyacrylamide gels. The differences in secondary structure and in the low molecular weight RNA species associated with 70S RNAs of these viruses indirectly implicate the 4 and 5S RNA in the maintenance of secondary structure of 70S RNA of Rous sarcoma virus.