Vaccinia virus WR induces an immediate and rapid inhibition of HeLa S3 cell RNA synthesis as determined by pulse-labelling with [3H]uridine. The inhibition was independent of the purity of the infecting virus preparation and the multiplicity of infection over the range of 4 to 200 pk.f.u./cell. Inhibition was not evident in cells pre-treated with cycloheximide or following infection with u.v.- or heat-inactivated virus, suggesting that viral protein synthesis was required. There was no apparent selective inhibition of any particular species of RNA. Following infection, the uptake of [3H]uridine into cellular pools and the subsequent biosynthesis of UTP proceeded at the same rate as in mock-infected control cells. The rate of degradation of pre-labelled RNA was not enhanced in infected cells compared to controls. Analysis of the nuclear DNA-dependent RNA polymerase (EC 2.7.7.6) activities revealed a progressive and eventually total loss of RNA polymerase B activity, no obvious effect on RNA polymerase A and the presence of a viral RNA polymerase, the possible significance of which is discussed.