The biological effects of many environmental toxins are mediated by genes containing Per-Arnt-Sim (PAS) domains, the aryl hydrocarbon receptor (AhR), and AhR nuclear translocator. Because these transcription factors interact with other PAS genes that form the circadian clockworks in mammals, we determined whether targeted disruption of the clock genes, Per1 and/or Per2, alters toxin-induced expression of known biological markers in the AhR signaling pathway. 2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD), a prototypical Ahr agonist, had an inductive effect on mammary gland expression of cytochrome P450, subfamily I, polypeptide 1 (Cyp1A1) mRNA regardless of genotype. However, TCDD-mediated Cyp1A1 induction in the mammary glands of Per1(ldc) and Per1(ldc)/Per2(ldc) mice was significantly (17.9- and 5.9-fold) greater than that in wild-type (WT) animals. In addition, TCDD-induced Cyp1B1 expression in Per1(ldc) and Per1(ldc)/Per2(ldc) mammary glands was significantly increased relative to that in WT mice. Similar to in vivo observations, experiments using primary cultures of mammary gland tissue demonstrated that TCDD-induced Cyp1A1 and Cyp1B1 expression in Per1(ldc) and Per1(ldc)/Per2(ldc) mutant cells was significantly greater than that in WT cultures. AhR mRNA levels were distinctively elevated in cells derived from all mutant genotypes, but they were commonly decreased in WT and mutant cultures after TCDD treatment. In WT mice, an interesting corollary is that the inductive effects of TCDD on mammary gland expression of Cyp1A1 and Cyp1B1 vary over time and are significantly greater during the night. These findings suggest that clock genes, especially Per1, may be involved in TCDD activation of AhR signaling pathways.