The rate of recombination of nitrogen atoms by three-body collision has been determined at room temperature and 0.5–1.3 mm pressure. The nitrogen was dissociated (about 3–6% nitrogen atoms) in a condensed discharge and pumped continuously through a glass tube 4.2 cm i.d. and 2.5 m long. The atom concentration was measured at a given point by ``titrating'' the N atoms with NO which was introduced into the gas stream at this point which was a short distance from the discharge tube. The N atoms react essentially instantaneously with NO in this titration, N+NO→N2+O.When excess NO is added, the unconsumed NO combines with the O atoms formed yielding the typical oxygen afterglow. If insufficient NO is added, however, the unconsumed nitrogen atoms react with the O atoms formed, yielding the typical blue color of the NO bands, N+O+M→NO*+M,When the quantity of NO added essentially equals that of the N atoms, the gas is practically colorless. This gives the N atom concentration at this point. In order to determine the rate of recombination of the N atoms, the relative decrease in intensity of the yellow nitrogen afterglow was obtained by following the reaction along the tube by means of a photomultiplier. The intensity of the afterglow is known to be proportional to the square of the N atom concentration. With the decrease of N atoms as a function of time and with the absolute N atom concentration known at a given point, the rate coefficient could be determined. The over-all value of the rate coefficient for the recombination reaction, N+N+M→N2+M,was determined by this experimental method to be k=1.72(±0.17)×10−32cm6/molecules2sec. The third body was nitrogen or argon; the rate coefficient was the same for both gases within the limits of error given.