According to statistical data of the WHO, about 4% of human population suffer from some form of epilepsy. Earliest paroxysms of epileptic seizures are appeared in age before 20 years-old in more than 50% of patients. In some types of the childhood epilepsy, spontaneous cessation of the illness is observed during maturation, and one suppose that fundamental mechanisms of childhood epilepsy can be explained by some peculiarities of developing brain. However, among experimental models elaborated for studying of epileptogenic mechanisms, investigations in adult animals prevail. In this study, an investigation of some peculiarities of epileptiform activity in the brain of developing rat pups have been carried out on a model of epilepsy provoked by 4-Aminopyridine (4-AP), blocker of potassium channels. In experiments with chronic animals, intracortical microinjections of 4-AP in dosage range from 0.2 to 1.5 L of 25 mM solution were performed in rats from 19 to 40 days postnatal, with subsequent ECoG recordings. The results of the experiments have shown a number of differences in characteristics of spike-wave discharges (SWDs) in rat pups 18-25 day-old, in a comparison to rats 1month-old or older. In the youngest rats, paroxysms of SWDs with internal frequency not higher than 6 Hz were observed while in older pups SWD frequencies have increased up to 12-18 Hz. More short latent periods of SWD appearance (3-4 min) were typical, whereas latency in older rats was so long as 20-25 min. Total duration of collected SWD activity in ECoG of 20-25 day-old animals was about 60% from whole recording time in 20 min after 4-AP microinjection, whereas in older rats it was only about 20% from whole recording time during 1.5 hour-long period. In the youngest animals, convincing motor seizures were not observed: during SWD paroxysm local "wet dog shakes" and movements arrest have appeared. It seems, an immaturity of neuronal networks, particularly in cortico-thalamic system, in rat pups didn’t allowed to maintain high-frequency SWD seizures in response to 4-AP injection, which are usual events in adult rats. More short latency as well as more prolonged total duration of epileptiform activity in young rats indicate on immaturity of intracortical inhibitory mechanisms. Data obtained demonstrate significant peculiarities of synchronization of neuronal activity, which are typical ones in developing brain. Subsequent investigations of features of the brain processes in early postnatal animals can be very prospective way for better understanding of fundamental mechanisms of both childhood and juvenile forms of epilepsy.