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Tunneling ionization is one of the fundamental processes for molecules in intense laser fields, and the tunnel- ionized molecules are in various electronic states. Here, we report an experimental study of the population dynamics of N2+(B²Σu+) and N2+ (Y²Σg+) generated in intense femtosecond laser fields by simultaneously measuring the fluorescence emission spectra and the transmission spectra. The results reveal that the population inversion exists between N2+(B B²Σu+) and N2+(X²Σg+). But the population difference is much smaller than the absolute population of N2+(B²Σu+). The present study provides insight into understanding the observation of the generation of an air laser from the tunnel-ionized nitrogen molecules, which has been attributed to the population inversion between N2+(B²Σu+) and N2+(X²Σg+) in intense femtosecond laser fields at 800-nm wavelengths. [ABSTRACT FROM AUTHOR]