Intracellular recordings and current and single-electrode voltage-clamp techniques were used to study the membrane responses of CA1 pyramidal neurons to bath application of L-homocysteic acid (L-HC) in the rat hippocampal slice preparation. In control artificial cerebrospinal fluid (ACSF), L-HC (25 - 250 μM) depolarized the membrane and induced a burst-like firing pattern. Both the membrane depolarization and the burst firing were blocked by the N-methyl-D-aspartic acid (NMDA) receptor antagonists D-(-)-2-amino-5-phosphonovaleric acid (AP-5, 50 μM), D-(-)-2-amino-7-phosphonoheptanoic acid (AP-7, 50 μM) and (±)-3-(2-carboxy-pipera-zin-4-yl)-propyl-1-phosphonic acid (CPP, 20 μM). In ACSF containing tetrodotoxin (1 μM), L-HC (100–300 μM) induced at resting membrane potential a depolarization which was associated with a small increase in input conductance. These effects were unaffected by 6-cyano-7-nitroquinoxaline-2, 3-dione (CNQX, 10–20 μM) but were fully blocked by AP-5, AP-7 (50 μM) and CPP (10–20 μM). In voltage-clamp experiments, L-HC induced slow inward currents which were voltage-dependent between - 70 and - 30 mV and reversed polarity near 0 mV. The L-HC-induced inward current was unaffected by CNQX (10–20 μM) but was strongly reduced by AP-5 or AP-7 (50 μM). The L-HC-induced inward current was temperature-dependent. Between -60 and -70 mV, its amplitude increased by 320% when the temperature was lowered from 33 to 22°C. The L-HC-induced current was also potentiated by the specific L-HC uptake blocker β-p-chlorophenylglutamate (Chlorpheg, 0.5–2 mM). These data suggest that L-HC preferentially activates NMDA receptors in CA1 hippocampal neurons.