The formation of NO molecules during a single plasma pulse in a low-pressure dc discharge is measured using time resolved tunable diode laser absorption spectroscopy in the infrared region. The pulse duration ranges from 280 νs to 16 ms and the pulse current ranges from 20 to 80 mA. The gas pressure is 133 Pa. Experimental results show that NO density is about proportional to the product of the pulse current times the pulse duration. NO formation mechanisms are discussed. We show that reaction of oxygen atoms with vibrationally excited nitrogen molecules (N₂(X, v > 12) + O) does not impact the NO concentration. Numerical computation of a simplified kinetics taking into account excited metastable state N₂(A) for the NO formation shows good agreement.