The escape rate of nitrogen from Mars is calculated to be 2.3¿105 s-1 for low solar flux conditions and 8.9¿105 s-1 for high solar flux conditions. The major source of energetic atoms is dissociative recombination of ground state and vibrationally excited N2+ ions. The measured 15N/14N isotope ratio can be reproduced by postulating an early dense atmosphere during which little differentiation occurred.