Reovirus cores contain an RNA transcriptase capable of synthesizing messenger RNA. When cores are suspended in 1 X SSC at 37 degrees they are quiescent and synthesize no product, but in the presence of the components of an RNA transcriptase reaction mixture they actively synthesize mRNA. Photochemical crosslinking has been used to investigate the arrangement of RNA and protein in both "quiescent" and "active" cores. Irradiation induces the formation of a noncovalent RNA:protein complex in "quiescent" but not in "active" cores. This difference is attributed to a conformational change in the reovirus core which results from the transition between the "quiescent" and "active" states of the particle.