A transient (10 min) exposure of brain slices of young adult rats to elevated extracellular calcium (5 mM) resulted in a long-lasting potentiation of field potentials evoked in layer II/III and layer V horizontal connections of the primary motor cortex. This form of synaptic plasticity was blocked by D,L-2-amino-5-phosphonovalerate (APV, 100 muM), an antagonist of NMDA receptors. (C) 2002 Elsevier Science B.V. All rights reserved.