The soybean phytoestrogen, genistein, is increasingly consumed as an alternative therapeutic for age-related diseases. The aim of this study was to examine the morphofunctional characteristics of adrenocorticotrophic (ACTH) cells and blood concentrations of ACTH in sham-operated, orchidectomized and genistein-treated orchidectomized, 16-month-old Wistar male rats. Genistein (10 mg/kg/day) was administered subcutaneously for three weeks, while the control groups received the vehicle alone. Orchidectomy and genistein treatment decreased the volume density of ACTH cells and reduced (p < 0.05) circulating ACTH concentrations in comparison with control groups. In conclusion, genistein modulated the morphofunctional features of ACTH cells and decreased blood ACTH levels.