Vorozole, a selective aromatase inhibitor, was administered in ovo to test the specific embryonic role of estrogen in conferring the sex distinction in GH release and body phenotype in broilers. On Day 6 of incubation, eggs were injected with saline or with different concentrations of vorozole. Postnatal blood samples were analyzed for T-3, T-4, GH, estradiol (E-2), and testosterone (T). At the age of 4 wk, control and vorozole-treated birds were cannulated, and serial blood samples were withdrawn every 10 min for 5 hr, wherein GH pulsatility characteristics were determined using deconvolution analysis. The proportional abdominal fat pad weight was reduced significantly in the treated groups, especially in female birds. The vorozole treatment increased plasma T-3, E-2, T, and GH concentrations, and decreased T-4. The frequency of the GH pulses was lower and the interval between the bursts (min) was higher in the vorozole-treated group, as were the mass secreted per burst (ng/ml), the amplitude (ng/ml/min) and the production rate (ng/ml/5 hr). In conclusion, early in ovo treatment with a potent aromatase inhibitor is able to increase the mean serum T-3 and GH concentration and masculinize the GH pulse pattern, resulting in an economically favorable decrease in abdominal fat pad content in male and female broilers at slaughter age. (C) Elsevier Science Inc. 1998.