We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope Imaging Spectrograph observations of the weak interstellar N I λ1160 doublet toward 17 high-density sight lines [N(H_(tot)) ≥ 10~(21) cm~(-2)]. When combined with published data, our results reveal variations in the fractional N I abundance showing a systematic deficiency at large N(H_(tot)). At the FUSE resolution (～20 km s~(-1)), the effects of unresolved saturation cannot be conclusively ruled out, although O I λ1356 shows little evidence of saturation. We investigated the possibility that the N I variability is due to the formation of N_2 in our mostly dense regions. The 0-0 band of the c′_4 ~1Σ_u~+-X ~1Σ_g~+ transition of N_2 at 958 A should be easily detected in our FUSE data; for 10 of the denser sight lines, N_2 is not observed at a sensitivity level of a few times 10~(14) cm~(-2). The observed N I variations are suggestive of an incomplete understanding of nitrogen chemistry.
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