Regulator of calcineurin 1 (RCAN1/MCIP1/DSCR1) regulates the calmodulin-dependent phosphatase calcineurin. Because it is located on human chromosome 21, RCAN1 has been postulated to contribute to mental retardation in Down syndrome and has been reported to be associated with neuronal degeneration in Alzheimer's disease. The studies herein are the first to assess the role of RCAN1 in memory and synaptic plasticity by examining the behavioral and electrophysiological properties of RCAN1 knock-out mice. These mice exhibit deficits in spatial learning and memory, reduced associative cued memory, and impaired late-phase long-term potentiation (L-LTP), phenotypes similar to those of transgenic mice with increased calcineurin activity. Consistent with this, the RCAN1 knock-out mice display increased enzymatic calcineurin activity, increased abundance of a cleaved calcineurin fragment, and decreased phosphorylation of the calcineurin substrate dopamine and cAMP-regulated phosphoprotein-32. We propose a model in which RCAN1 plays a positive role in L-LTP and memory by constraining phosphatase signaling.