The concentrations of the neutral active species in the afterglow of a nitrogen and helium atmospheric-pressure plasma have been determined by optical emission and absorption spectroscopy and by numerical modeling. For operation with 10 Torr N-2 and 750 Torr He, at 15.5 W/cm(3) rf power, 30.4 L/min flow rate, and a neutral temperature of 50degreesC, the plasma produced 4.8 x 10(15) cm(-3) of ground state nitrogen atoms, N(S-4), 2.1 x 10(13) cm(-3) of N-2(A (3)Sigma(u)), 1.2 x 10(12) cm(-3) of N-2(B (3)Pi(g)), and 3.2 x 10(9) cm(-3) of N-2(C (3)Pi(u)). The concentration of nitrogen atoms and metastable state nitrogen molecules, N-2(A), increased gradually, with the rf power and the nitrogen partial pressure. Both the model and experiments indicate that ground-state nitrogen atoms are the dominant active species in the afterglow beyond 2.0 ms.