1. Anomalous rectification occurred in 54 of 56 hippocampal CA1 neurons studied in vitro. This phenomenon is characterized by a progressive increase in input resistance with membrane depolarization. An average increase in membrane resistance of 45% occurred over a 15-mV region of membrane potential immediately subthreshold to cellular firing. 2. Both Mn2+, a Ca+ antagonist, and tetrodotoxin (TTX), a neurotoxin that blocks regenerative Na+ currents, eliminated anomalous rectification. Ba2+, which can both contribute to intracellular cation influx as well as reduce K+ conductance, increased the magnitude of anomalous rectification. The observations are indirect evidence indicating that a Ca2+-Na+ current may produce the inward-going rectification. 3. Enhancement of anomalous rectification by Ba2+ was associated with the onset of membrane oscillations and spontaneous bursts of repetitive discharges. The magnitude of anomalous rectification may be one factor that predisposes some cortical neurons to bursting behavior.