We have cloned via recombinant DNA technology the mRNA sequence of rat pancreatic ribonuclease, and have determined the entire nucleotide sequence of the mature message. Clones bearing RNase sequences within a double-stranded complementary DNA library of rat pancreatic mRNA were initially detected by hybridization with size-fractionated rat pancreatic polyadenylated RNA that included mRNA 0.85 to 1.0 kilobase in length. Recombinant plasmids bearing RNase mRNA sequences were conclusively identified by comparison of the amino acid sequence of the encoded protein with the known amino acid sequence of rat RNase. RNase mRNA is 783 nucleotides in length, plus a poly(a) tail with an average length of 140 nucleotides, and contains long 5' and 3' noncoding regions relative to other pancreatic mRNAs. It encodes a secretory preRNase of 152 amino acid residues including a signal peptide of 25 amino acids.