We are investigating the mechanisms by which aromatic hydrocarbons, such as 3-methylcholanthrene (MC), suppress hepatic cytochrome P450 2C11 (CYP2C11) gene expression. CYP2C11 is an enzyme expressed in the liver of male rats and is regulated by a pulsatile pattern of GH secretion. We have previously shown that MC attenuates the stimulatory effect of GH on CYP2C11 expression in hypophysectomized male rats. In follow-up studies we evaluated the effect of MC on GH-stimulated signal transducer and activator of transcription 5b (STAT5b) phosphorylation, nuclear translocation, and DNA-binding activity. GH-stimulated increases in hepatic nuclear STAT5b and phospho-STAT5b levels were not different between groups of hypophysectomized rats receiving MC or vehicle. This observation was corroborated at the DNA-binding level by EMSA. We also measured GH-induced STAT5b activation in the H4IIE rat hepatoma cell line. STAT5b DNA-binding activity detected in GH-treated cells was not affected by MC. Immunocytochemistry experiments revealed no effect of MC on GH-stimulated STAT5b nuclear translocation in H4IIE cells. These in vivo and in vitro data suggest that interference with GH-stimulated STAT5b activation does not constitute a mechanism by which MC attenuates the stimulatory effect of GH on CYP2C11 gene expression.