Early in ontogeny, various parts of the nervous system, including the hippocampus of newborn rats, generate a special rhythmic activity, giant depolarizing potentials [1]. The latter are supposed to play an important role in the body growth and connection formation [2]. Giant potentials are accompanied by synchronous excitation of a large group of cells [3], which testifies to an increased neuronal activity in the hippocampus of newborn animals.When neuronal activity is high, adenosine triphosphate (ATP) is released into extracellular space to modulate synaptic transmission [4]. Histochemical analysis showed the existence of P2X2, P2X4, and P2X6 receptors in the newborn rat hippocampus [5]. Recently, ATP and adenosine were shown to modulate the frequency of giant depolarizing potentials and spontaneous postsynaptic potentials in the newborn rat hippocampus via P2X-subclass andenosine A1 and, presumably, ATP receptors [6,7]. However, the effects of many antagonists and agonists of purinergic receptors are nonspecific, which makes it difficult to evaluate the involvement of specific subtypes of purinergic receptors. The purpose of this study was to determine the role of P2X purinergic receptors in the modulation of giant depolarizing potentials.Sections of the hippocampus of two-to six-day-old rats ( n = 8) were made. The rats were preliminarily anesthetized with an intraperitoneal urethane injection (2 g/kg body weight). Sections (500 µ m thick) were obtained using a Leica VT1000S vibratome (Germany). Dissection was conducted with the use of a cooled artificial cerebrospinal fluid (ACSF) that contained (mM): NaCl, 130; KCl, 3.5; NaH 2 PO 4 , 1.2; NaHCO 3 , 25; MgCl 2 , 1.3; CaCl 2 , 2; glucose, 25. The fluid was oxygenized with carbogen (95% O 2 and 5% CO 2 ) (pH 7.3-7.4). After 1 h of incubation of the sections in ACSF at room temperature, the cellular and network activities were recorded using the patch-clamp method in the whole-cell configuration as described previously [7]. The patch-clamp electrodes were made from thin borosilicate glass; their resistance was 5 − 7 M Ω when filled with a solution of the following com-(a) (b) (c) 20 mV 10 s Fig. 1. Changes in the frequency of giant depolarizing neuronal potentials of the hippocampal NA3 field after the application of 50 µ M TNP-ATP and changes four minutes after washing of the latter. Records: (a) spontaneous activity in the control; (b) after TNP-ATP application; (c) after washing.