Objective To explore the possible mechanisms that cause the dentate gyrus (DG) neurons to play different roles in information coding. Methods In vivo extracellular single unit recording was performed on 22 waking female guinea pigs, which were positioned in a sound-attenuated recording chamber without any muscular relaxants. The spontaneous firing patterns of the DG neurons were detected and compared. Results There were two different electrophysiological populations in the DG of guinea pigs, principal cells (PCs) and fast spiking interneurons (INs). Of the PCs, 1.3% discharged regularly, 48.1% irregularly and 50.6% in bursts; in contrast, of the INs units, 64.1% discharged regularly, 2.6% irregularly and 33.3 % in bursts. The spontaneous firing patterns of PCs were significantly different from those of INs (P <0.01). In addition, the differences of several interspike interval (ISI) parameters also have been observed: (1) the ISI coefficients of variation of PCs (3.39 +/- 3.56) were significantly higher than those of INs (1.08 +/- 0.46) (P < 0.01 ); (2) the ISI asymmetric indexes of PCs (0. 047 +/- 0. 059) were significantly lower than those of INs (0. 569 +/-0. 238) (P < 0.01). Conclusion In the DG, the spontaneous firing patterns of PCs were significantly different from those of INs. The former were prone to fire in bursts, the latter were prone to fire regularly. The different roles in information coding between PCs and INs might be caused by their different firing patterns.