1. The nucleotide and deduced amino acid sequences of rat liver MAO A were determined, and sequence identities among MAO A and B from rat, human and bovine were compared. 2. MAO A from rat exhibited greater than 85% sequence identity with bovine and human MAO A, and 70% identity with rat MAO B. 3. Rat adrenal cDNAs were restriction mapped, partially sequenced and found to be identical to rat liver MAO A, suggesting that these two tissues express the same polypeptide.