The adrenergic transmitter norepinephrine (NE) dramatically increases the prominence of only two out of the hundreds of [ 35S]methionine-labeled pineal proteins resolved by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE). One of these regulated proteins is AlP 37/6 (37 kDa, p I ∼ 6). The labeling of this protein is increased ∼ 100-fold by NE. In the study presented here the identity of AIP 37/6 was investigated. The results of microsequencing, immunochemical analysis of 2D-PAGE blots and size ecllusion chromatography indicate that AIP 37/6 is an isoform of cytosolic malate dehydrogenase (cMDH; ∼ 36.3 kDa; p I ∼ 6.5). Associated studies indicate that this isoform is phosphorylated whereas the bulk of cMDH is not. Cotranslational phosphorylation of cMDH is discussed.