The development of electroshock seizure threshold (EST) from 10 to 30 days postnatally and the pattern of maximal electroshock seizure (MES) at 25 days postnatally were studied in normal and neonatally adiothyroidectomized female rats. The results show EST in hypothyroid animals to be consistently and significantly lower than in controls, indicating higher brain excitability in thyroidectomized animals. The MES pattern shows significantly shorter flexion and longer extension in hypothyroid rats, suggesting that the capacity to sustain maximal activity is increased in these animals. Prolonged recovery time from maximal seizures and sustained excitation from minimal seizures was also observed in some hypothyroid animals. Our findings demonstrate that neonatal thyroidectomy impairs the maturation of cortical and/or subcortical structures involved in seizure activity and that the nature of the alteration depends upon the relative degree of maturation of the specific brain area affected.