The enzyme Ca2+ /calmodulin-stimulated protein kinase II (CaM-PK II) belongs to a family of calmodulin-stimulated serine/threonine protein kinases that are widely distributed in nature, but are especially enriched in the brain (1 –3 ). CaM-PK II is also found in a number of subcellular fractions, where it may be associated with specific binding proteins (3 ). CaM-PK II has a number of unique characteristics that allow it to be identified see Section 1.1.), and have its activity see Section 1.2.) and level determined (see Section 1.3.).