Neural activity regulates dendrite and synapse development, but the underlying molecular mechanisms are unclear. Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) is an important sensor of synaptic activity, and the scaffold protein liprinalpha1 is involved in pre- and postsynaptic maturation. Here we show that synaptic activity can suppress liprinalpha1 protein level by two pathways: CaMKII-mediated degradation and the ubiquitin-proteasome system. In hippocampal neurons, liprinalpha1 mutants that are immune to CaMKII degradation impair dendrite arborization, reduce spine and synapse number, and inhibit dendritic targeting of receptor tyrosine phosphatase LAR, which is important for dendrite development. Thus, regulated degradation of liprinalpha1 is important for proper LAR receptor distribution, and could provide a mechanism for localized control of dendrite and synapse morphogenesis by activity and CaMKII.