The effect of the trypanocidal dyes acriflavine and ethidium bromide on the sedimentation pattern of tritium-labelled RNA of the sciatic nerve of chicks was studied. Tissues from 14-day embryos, 17-day embryos, and 3-day chicks were incubated with [5-3H]uridine in organ culture, in the presence and in the absence of the inhibitors. Acriflavine and especially ethidium bromide caused a selective inhibition of the labelling of the RNA species which sedimented between the 17.2 S and 5.8 S components of the bulk RNA in the older animals. The labelling of the RNA with sedimentation values higher than 17.2 S was not decreased by ethidium bromide during a 1.5 h incubation period, indicating that rRNA, rRNA precursors, or heterogeneous nuclear RNA was not affected by the drug.