Limited proteolysis converts the 39200 molecular weight catalytic domain of rat tyrosine hydroxylase to a monomer with a molecular weight of 37600. The purified monomer is almost fully active, with minor changes in kinetic parameters at pH 7. Mass spectral analysis and N-terminal sequencing of the proteolytically generated species establish that 20 amino acids have been removed from the carboxyl terminus and five from the amino terminus. Based on these results, the carboxyl terminus is responsible for tetramer formation by tyrosine hydroxylase. The sequence of amino acids which is removed is consistent with a coiled coil structure in the intact tetramer.