Calcineurin is a Ca 2 + /calmodulin-regulated protein phosphatase which is thought to play an essential role in the intracellular Ca 2 + -signal transduction. The present study showed that calcineurin immunolabeling was differentially concentrated in the two distinct compartments showing a mosaic-like pattern in the adult cat striatum. The compartment of heightened calcineurin immunolabeling corresponded to the extrastriosomal matrix visualized by calbindin-D 2 8 K immunostaining. Under the light microscope, striatal neurons appeared to be less strongly immunoreactive for calcineurin in the striosomes than in the matrix compartment. These findings suggest that the novel striatal compartments may differ in the intracellular Ca 2 + -signaling cascade associated with protein dephosphorylation.