The ligand activated basic-helix-loop-helix (bHLH)-PAS transcription factor, the aryl hydrocarbon receptor (AHR) protein, heterodimerizes with its partner protein the aryl hydrocarbon receptor nuclear translocator (ARNT). The heterodimer activates transcription via xenobiotic responsive elements to regulate the transcription of a battery of biotransformation genes as well as genes involved in growth, differentiation, and cellular homeostasis. In this study we have investigated the diurnal expression of cytochrome P450 1A1, one of the genes in the AHR target gene battery, in rat pituitary and liver. The mRNA expression patterns of AHR, ARNT, and the periodic gene (PER2) were also analyzed. PER2 belongs to another group of bHLH-PAS transcription factor complexes, which are involved in the control of circadian rhythms. Diurnal variation of cytochrome P450 1A1 (CYP1A1) mRNA expression was observed in the anterior and posterior pituitary and in the liver. The accumulation of CYP1A1 mRNA occurred during different times of the day and exhibited an opposite expression in anterior and posterior pituitary, respectively. A daily upregulation of CYP1A1 and PER2 mRNAs that was in antiphase to the AHR and ARNT mRNAs was seen in the liver. The AHR/ARNT system is considered a defense system against toxic chemicals. The high inducibility of CYP1A1 in the pituitary, shown in an earlier study, as well as the tissue specific expression patterns shown here, suggest that AHR and CYP1A1 may play a physiological role in controlling neuroendocrine functions.