The nitric oxide synthases (NOS) catalyze a two‐step oxidation of l‐arginine (Arg) to generate NO. In the first step, O2 activation involves one electron being provided to the heme by an enzyme‐bound 6R‐tetrahydro‐l‐biopterin cofactor (H4B), and the H4B radical must be reduced back to H4B in order for NOS to continue catalysis. Although an NADPH‐derived electron is used to reduce the H4B radical, how this occurs is unknown. We hypothesized that the NOS flavoprotein domain might reduce the H4B radical by utilizing the NOS heme porphyrin as a conduit to deliver the electron. This model predicts that factors influencing NOS heme reduction should also influence the extent and rate of H4B radical reduction in kind. To test this, we utilized single catalytic turnover and stop‐freeze methods, along with electron paramagnetic resonance spectroscopy, to measure the rate and extent of reduction of the 5‐methyl‐H4B radical formed in neuronal NOS (nNOS) during Arg hydroxylation. We used several nNOS variants that supported either a slower or faster than normal rate of ferric heme reduction. We found that the rates and extents of nNOS heme reduction correlated well with the rates and extents of 5‐methyl‐H4B radical reduction among the various nNOS enzymes. This supports a model where the heme porphyrin transfers an electron from the NOS flavoprotein to the H4B radical formed during catalysis, revealing that the heme plays a dual role in catalyzing O2 activation or electron transfer at distinct points in the reaction cycle.