Alkylphenols such as 4-nonylphenol (NP) are one of the wide variety of environmental chemicals reported to have estrogenic effects in both in vitro and in vivo studies. Induction of eggshell zona radiata proteins (Zrp) and vitellogenin (Vtg) mRNA and protein synthesis in the liver are widely used biomarkers for xenoestrogen exposure in fish. However, little work has been done to characterize the molecular effects of xenoestrogens on other potential target organs such as the pituitary. To evaluate pituitary effects and develop new potential biomarkers for xenoestrogens, the influences of NP and 17beta-estradiol (E2) on the mRNA levels of pituitary gonadotropic hormone (GTH) beta subunits [leutinizing hormone beta (LH beta or GTH II beta) and follicle stimulating hormone beta (FSH beta or GTH I beta)], prolactin (PRL), growth hormone (GH) and the pituitary specific transcription factor (Pit-1) were investigated in individual male and female juvenile Atlantic salmon (Salmo salar), 3 days after a single intraperitoneal (i.p.) injection. In one experiment, fish were injected with NP (125 mg/kg body weight (BW)) or E2 (5 mg/kg BW) and a semi-quantitative reverse transcriptase-polymerase chain reaction (RT-PCR) method was used to analyze LH beta and FSH beta mRNA levels. In the second experiment, fish were injected with three doses of NP (10, 50, 125 mg/kg BW) or a single dose of E2 (5 mg/kg BW) and Northern blot analysis was used to quantify LH beta, FSH beta, PRL, GH and Pit-1 mRNAs. Both NP (50 and 125 mg/kg BW) and E2 significantly induced LH beta mRNA levels (P<0.01), but only in females. The highest dose of NP (125 mg/kg BW) significantly induced Pit-1 mRNA in males (P<0.01). NP did not have significant effects on any of the other pituitary transcripts. NP induced LH beta mRNA synthesis in females by up to 6-fold and the changes appeared to correlate with the increases in hepatic Vtg and Zrp mRNA levels. The results show that LH beta mRNA assay in female juvenile salmonids may be used as a marker for pituitary effects of xenoestrogens. The data also suggest that NP may have the potential to perturb the regulation of LH beta gene expression by mimicking E2.