The sequence complexity of heterogeneous nuclear RNA is sea urchin gastrulas was measured by RNA-driven hybridization reactions with nonrepetitive sea urchin DNA. 28.5% of the sequence complexity of the genome is represented in the nuclear RNA. This amounts to 1.74 X 10(8) nucleotides of diverse sequence, more than 10 times the nucleotide complexity of the polysomal messenger RNA extracted from sea urchin embryos at the same stage. The complex set of nuclear RNA sequences driving this hybridization reaction was shown to be the same as the rapidly labeled hnRNA, using pulse-labeled nuclear RNA as driver.