BACKGROUND: Adult rats undergo five distinct electrographic stages during status epilepticus (SE). Whether developing animals manifest those stages is not yet known. GOALS: Determine in the kainic acid (KA) model: (1) the EEG stages of SE in P15 and P35 rats; (2) the relative susceptibilities of these two age groups to develop SE; and (3) the effect of phenobarbital on SE stages. MATERIALS AND METHODS: Experiment 1: Three groups of P15, and three of P35 rats received intraperitoneally (i.p.) low (5 mg/kg), intermediate (10 mg/kg), or high (15 mg/kg) KA doses. Experiment 2: One group of P35 rats received KA (12 mg/kg), and one KA and phenobarbital (70 mg/kg i.p.). EEGs were recorded through intracranial electrodes and were reviewed and staged blindly. RESULTS: Both age groups manifested the five EEG stages of SE, but these occurred at the low dose in the P15 rats, and at the intermediate and high doses in the P35 rats. Unlike P35 rats, P15 rats were less likely to progress through all five stages, and had different behavioral manifestations that did not segregate into distinct stages. Phenobarbital caused an initial increase in paroxysmal beta discharges and in tonic activity and scratching. It subsequently resulted in less severe and shorter stages of SE. CONCLUSION: Both P15 and P35 rats can progress through the five distinct electrographic stages of SE. However, P15 rats are less likely to progress through all these stages. They also have a lower seizure threshold and different behavioral manifestations that do not segregate into separate stages. Phenobarbital shortens and modifies the behavioral and electrographic stages of SE.