Penetrating cortical trauma frequently results in delayed development of epilepsy. In the rat undercut model of neocortical posttraumatic hyperexcitability, suppression of neuronal activity by exposing the injured cortex to tetrodotoxin (TTX) in vivo for approximately 2 weeks prevents the expression of abnormal hypersynchronous discharges in neocortical slices. We examined the relationship between neuronal activity during the latent period after trauma and subsequent expression of hyperexcitability by varying the timing of TTX treatment. Partially isolated islands of rat sensorimotor cortex were treated with Elvax polymer containing TTX to suppress cortical activity and slices obtained for in vitro experiments 10 to 15 days later. TTX treatment was either started immediately after injury and discontinued after a variable number of days or delayed for a variable time after the lesion was placed. Immediate treatment lasting only 2 to 3 days and treatment delayed up to 3 days prevented hyperexcitability. Thus, there is a critical period for development of hyperexcitability in this model that depends on cortical activity. We propose that the hyperexcitability caused by partial cortical isolation may represent an early stage of posttraumatic epileptogenesis. A hypothetical cascade of events leading to subsequent pathophysiological activity is likely initiated at the time of injury but remains plastic during this critical period.