We have compared the transcriptional complexity of nuclear and cytoplasmic RNAs from various rat tissues and tumor lines. Work by others had suggested that 4-10X more unique sequence DNA is transcribed in the mammalian brain than in other organs. We have extended these observations to rat tissues using RNA driven hybridization to iodinated unique sequence rat DNA. At apparent hybridization saturation, brain cytoplasmic RNA anneals to 10-11 percent of unique sequence DNA, which corresponds to 20-22 percent of the unique sequence genome information, assuming asymmetric transcription. Half of this complexity is found in polyadenylated (poly A ion) RNA species, the other half in nonpolyadenylated RNAs. Additivity experiments suggest that the poly A positive and the Poly A negative RNA fractions contain nonoverlapping sets of sequences. Cytoplasmic RNAs from the liver, kidney, several independently isolated rat neural tumor lines, and a HTC hepatoma line transcribe 4-7 percent of their unique sequence information. These data suggest that 3-5X more single copy DNA is transcribed into cytoplasmic RNA in the brain compared other tissues and clonal cells lines, including those derived from neural and glial tumors. Hybridization of nuclear RNA from brain saturates about 15-16 percent of the iodinated probe DNA, corresponding to transcription from 30-32 percent of the single copy genome. Nuclear liver RNA shows relatively high complexity, saturating 22 percent of asymmetrically transcribed unique sequence DNA, while kidney and spleen nuclear RNA both saturate about 10-12 percent. The nuclear RNA sequences from liver, kidney and spleen appear to be included withing the diversity of brain nuclear RNA. BIBLIOGRAPHIC REFERENCE: Lindstrom, D.M., Deeb, S. and Sueoka, N. "Transcriptional Complexity of rat tissues and tumor lines." J. Cell Biol. 70, 342a (1976) Abstract.