Using an exact multifrequency algorithm, relaxation rates for individual 15-μ CO2 lines have been calculated in the upper stratosphere and loweer mesosphere for the Cira (1972) global mean reference atmosphere. The relaxation rate dependence on wave number is obtained and examined as a function of line strength and altitude, to clarify the underlying physics. Wave numbers n≲2.09 km-1 are accurately treated by the algorithm. The most rapid growth of relaxation rate with wave number is &tgr;-1 ∝ n1.2, which occurs for strong, mostly Doppler-broadened lines in the mesosphere. Overall relaxation rates for the CO2 15-μ band complex are obtained from an approximate model of the line strength distribution; these rates corroborate previous results of Fels. Our results for individual lines, however, differ significantly from Fels' parameterization. Comparison of the present results with our previous rates obtained with the diffusion approximation demonstrates reasonably good agreement down to wavelengths of ~8 km, below which this approximation seriously overestimates the relaxation rate.