We have recently isolated a rat homologue of the Caenorrhabditis elegans lin-10 product. Although rat lin-10 is expressed in the cytosol and membrane fractions of various tissues, it is distributed only in the membrane fraction in brain where it is enriched in the synaptic plasma membrane and postsynaptic density fractions. We have isolated here a rat lin-10-interacting protein from rat brain and identified it to be neurabin-II/spinophilin, which has recently been isolated as a protein interacting with protein phosphatase I and F-actin. Neurabin-II/spinophilin is ubiquitously expressed but enriched in brain, especially in the synaptic plasma membrane and postsynaptic density fractions. We discuss the physiological significance of the interaction of rat lin-10 with neurabin-II/spinophilin.