Heme oxygenases (HOs) catalyze the degradation of heme to biliverdin, carbon monoxide (CO), and free iron. The two major isoforms, HO-1 (inducible) and HO-2 (constitutive), are involved in a variety of physiological functions, including inflammation, apoptosis, neuromodulation, and vascular regulation. Major tools used in exploring these actions have been metalloporphyrin analogs of heme that inhibit the HOs. However, these tools are limited by their lack of selectivity; they affect other heme-dependent enzymes, such as cytochromes P450 (P450s), soluble guanylyl cyclase (sGC), and nitric-oxide synthase (NOS). Our laboratory has successfully synthesized a number of nonporphyrin azole-based HO inhibitors (QC-xx) that had little or no effect on sGC and NOS activity. However, their effects on various P450 isoforms have yet to be fully elucidated. To determine the effects of the QC-xx inhibitors on P450 enzyme activity, microsomal preparations of two rat P450 isoforms (2E1 and 3A1/3A2) and two human P450 supersome isoforms (3A4 and 2D6) were incubated with varying concentrations of HO inhibitor, and the activity was determined by spectrophotometric or fluorometric analysis. Results indicated that some QC compounds demonstrated little to no inhibition of the P450s, whereas others did inhibit these P450 isoforms. Four structural regions of QC-xx were analyzed, leading to the identification of structures that confer a decreased effect on both rat and human P450 isoforms studied while maintaining an inhibitory effect on the HOs.