In male rats androgens are involved in the regulation of follicle-stimulating hormone (FSH) synthesis and secretion. Two nonsteroidal antiandrogens, flutamide and Casodex, were used to study the influence of androgens on the carbohydrate structure of FSH isoforms and the relationship with their bioactivity in prepubertal male rats. Different doses of flutamide or Casodex (vehicle, 1, 5, or 10 mg/rat/day) were administered subcutaneously for 10 days to 23-day-old rats. Immunological FSH was determined by radioimmunoassay and the bioactivity by in vitro Sertoli cell bioassay. Concanavalin A affinity chromatography was used to study the distribution of immunoactive and bioactive pituitary FSH isoforms. A significant depletion of immunological and biological pituitary FSH contents was observed even at the lowest dose of flutamide or Casodex used. The bioactive/immunoactive ratio of pituitary FSH was reduced at the highest dose of flutamide; however, no change was observed in Casodex-treated rats, suggesting a differential effect of the antiandrogens on the FSH bioactivity. Flutamide treatment provoked a significant decrease in proportion and bioactivity of FSH isoforms bearing biantennary and truncated hybrid oligosaccharide side chains and an increase in the proportion but a decrease in bioactivity of FSH isoforms bearing high-mannose oligosaccharides. Conversely, Casodex administration did not modify the proportions of FSH isoforms, although those bearing biantennary and truncated hybrid structures were less bioactive, while those bearing high-mannose oligosaccharides were more bioactive. The highest dose of flutamide decreased the bioactive/immunoactive ratio of FSH isoforms with a high degree of branching in their carbohydrate chains. Our results suggest that androgens, acting directly and indirectly at the pituitary, regulate the selective incorporation of sugar residues to the FSH molecule, thus modulating its biological activity.