Aminoethylcysteine ketimine is a sulfur-containing cyclic compound produced by the enzymatic alpha-deamination of the parent aminoethylcysteine that has been detected in bovine brain and cerebellum. Aminoethylcysteine ketimine is known to dimerize spontaneously and easily lose one carboxyl group. This decarboxylated compound, simply named the dimer, has been recently detected in normal human urine. In this article we provide evidence on the occurrence of the dimer in the bovine cerebellum.