The zero-temperature transition pressure from molecular to monatomic nitrogen is predicted to be less than about 1 Mbar, and should be observable in diamond-anvil cells. This estimate is obtained from total-energy calculations in which compressed molecular nitrogen is found to dissociate into a simple cubic monatomic structure, slight distortions of which are common to other group-V elements. The calculated 35% volume change raises the possibility of a large barrier to dissociation and extensive regions of metastability.