This investigation tested the hypothesis that the degree of pyridoxine depletion rather than the status of neuronal maturity determines seizure proneness in the pyridoxine-deficient rat. Dietary pyridoxine deficiency was induced in neuronally mature rats. Seizure activity was monitored using computerized EEG analysis. Dietary pyridoxine deficiency of 10 weeks' duration induced in neuronally mature rats led to spontaneous convulsive seizure activity. Even moderately pyridoxine-deficient adult rats (on the deficient diet for less than 8 weeks) exhibited seizure-like diffuse spike and wave activity and electrocortical inhibition. Picrotoxin-, pentylenetetrazol-, or domoic acid-induced seizure thresholds were significantly reduced in pyridoxine-deficient rats when compared with normal controls. Pyridoxine-deficient rats exhibited increased dominance of delta and theta activities and increased hemispherical asymmetries in response to convulsant treatment.