Commissural CA3-CA3 (cCA3) long-term potentiation (LTP) was investigated in the anesthetized rat treated with the highly selective NMDA-receptor antagonist D,L-3[(+/-)-2-carboxypiperazin-4-yl]- propyl-1-phosphonic acid (CPP). Intraperitoneal injections of CPP did not significantly affect baseline test responses for either field EPSP slope or amplitude measures but did reduce LTP in a dose-dependent manner, with 3.2 mg/kg as the lowest effective dose. EPSP variability following tetanization was also significantly reduced in both the 3.2 mg/kg and 10.0 mg/kg groups. We interpret these results to suggest that a 3.2 mg/kg dose of CPP may be sufficient for studying the behavioral effects of this NMDA receptor antagonist.