Three neurotransmitters, namely adrenaline, serotonin and tryptamine inhibit the in vitro activity of several cytochrome P450 (CYP) isozymes (CYP1A2, CYP2C9, CYP2D6 and CYP3A). In order to test whether this effect is related to inhibition of the CYP-coupled NADPH reductase activity, we assayed the potential inhibitory effect of these neurotransmitters and their main metabolites on the NADPH reductase activity. Of the five compounds analyzed: tryptamine, tryptophol, serotonin, 5-hydroxytryptamine and adrenaline, only adrenaline significantly decreased NADPH reductase activity at the fixed concentration of 500 μM. However, the effect became negligible when adrenaline concentration was decreased to 100 μM: whereas a high inhibitory effect was observed in CYP2D6, CYP2C9 and CYP3A4 enzyme activities, the NADPH reductase activity remains unchanged. This study indicates that the effect of these endogenous neurotransmitters on CYP enzymes is not related to changes in the reductase activity. In the light of these findings further studies on the inhibitory effect of these neurotransmitters on CYP enzymes can be designed ruling out the modulation of the coupled NADPH reductase activity as a confounding factor.