We have previously shown that chronic developmental administration of N-methyl-D-aspartate (NMDA) antagonists reduces synaptic development; however, on withdrawal from NMDA antagonism, there is a rebound period during which synaptogenesis exceeds control levels. The current research was undertaken to explore this period of withdrawal, using the noncompetitive antagonist phencyclidine (PCP), examining 2 behavioral measures in which the NMDA receptor is implicated: 1. NMDA-induced seizures, and 2. learning and memory in the Morris water maze. Using a protocol identical to that previously used to examine synaptic development, male Long-Evans rats were given 1 daily SC injection of either 10 mg/kg PCP or its physiological saline vehicle for a period of 15 days, beginning on postnatal Day 5 (P5) and ending on P20. Animals were then assessed for either sensitivity to NMDA-induced seizures on P21, P26, P36, or P56, or they were assessed for their acquisition performance and initial heading in the Morris water maze on P23, P26, P30, P38, and P75. Chronic treatment with PCP resulted in greater behavioral ratings of seizure activity after NMDA administration, observed 1 (P21), 5 (P26), and 15 (P36) days after the last injection of PCP, indicating increased sensitivity of the NMDA receptor/channel complex during this period after withdrawal from developmental NMDA antagonism. PCP-treated animals also required significantly more trials to reach criterion in the Morris water maze on P23, P26, and P30, and displayed significantly less accurate initial swim headings on all test days. The results are discussed in terms of the role of the NMDA receptor-channel complex in development and learning/memory processes.