Asclepias subulataplant extract has previously demonstrated antiproliferative activity and antimutagenicity against heterocyclic aromatic amines (HAAs) commonly found in cooked meat. The objective of this work was to evaluate the in vitro ability of an ethanolic extract from the medicinal plantAsclepias subulataextract (ASE), non-heated and heated (180 °C), to inhibit the activity of CYP1A1 and CYP1A2, which are largely responsible for HAAs bioactivation. Ethoxyresorufin and methoxyresorufinO-dealkylation assays were performed in rat liver microsomes exposed to ASE (0.002–960 µg/mL). ASE exerted an inhibitory effect in a dose-dependent manner. The half inhibitory concentration (IC50) for unheated ASE was 353.6 µg/mL and 75.9 µg/mL for heated ASE in EROD assay. An IC40value of 288.4 ± 5.8 µg/mL was calculated for non-heated ASE in MROD assay. However, after heat treatment, the IC50value was 232.1 ± 7.4 µg/mL. Molecular docking of corotoxigenin-3-O-glucopyranoside, one of the main components of ASE, with CYP1A1/2 structure, was performed. Results show that the interaction of corotoxigenin-3-O-glucopyranoside with CYP1A1/2s’ α-helices, which are related with the active site and the heme cofactor, may explain the plant extract’s inhibitory properties. Results showed that ASE inhibits CYP1A enzymatic subfamily and may potentially act as a chemopreventive agent by inhibiting bioactivation of promutagenic dietary HAAs.