To further delineate the role of estradiol in the IGF system an experiment was conducted to determine the dosage of the aromatase inhibitor, anastrozole, needed to decreases serum concentrations of estradiol-17 (E2) in maturing boars. A second experiment was conducted to determine if administration of anastrozole to growing boars decreased serum concentrations of E2 and affected components of the serum and anterior pituitary gland (AP) IGF system vs untreated boars and barrows. In Experiment 1, 12 crossbred boars (292 days, 158 kg) were administered either 0, 1 or 10 mg/day anastrozole (n=4/group) beginning on day 1. Blood samples were collected every 7-14 days. Mean serum concentrations of E2 were decreased (P<0·05) in the 10 mg group vs the 0 and 1 mg groups by day 36; however, no difference (P>0·05) existed between the 0 and 1 mg groups. In Experiment 2, 24 crossbred boars and 12 barrows (101 days, 44 kg) were stratified by litter to one of three treatment groups (n=12): boars administered 10 mg/day anastrozole, boars administered 0 mg/day, and barrows administered 0 mg/day. Blood samples were collected and pigs were weighed on day 0 and every 14 days thereafter, then killed on day 84 when blood and APs were collected. The 10 mg/day pigs were fed the anastrozole-amended diet beginning on day 1. Mean serum concentrations of E2 did not differ (P>0·05) between the 10 mg/day pigs and 0 mg/day pigs on day 0; however, on day 15 through to 84 mean serum concentrations of E2 were greater (P<0·05) in 0 mg/day pigs than in the 10 mg/day pigs. Mean percentage increase in serum concentrations of IGF-I was greater (P<0·05) in untreated boars than anastrozole-treated boars and barrows from day 58 through to 84. Mean percentage of basal IGF-I increased (P<0·05) from day 29 through to 84 in untreated boars. Mean relative amounts of AP IGFbinding protein (IGFBP)-2 and -5 were less (P<0·01) in 10 mg/day pigs than in the 0 mg/day pigs, but each was greater (P<0·01) than in barrows administered 0 mg/day. These results indicate anastrozole administered at a dosage of 10 mg/day suppresses serum concentrations of E2 in pigs. Administration of anastrozole to boars reduced the percentage increase in serum concentrations of IGF-I and relative amounts of AP IGFBP-2 and -5. These data further support a role for E2 in regulating components of the IGF system in pigs.