FSH is secreted as a mix of isoforms with varying biologic attributes. To determine the functional significance of FSH heterogeneity, an acidic (ovine pituitary FSH; C-FSH) and less acidic mix (C-FSH exposed to neuraminidase; N-FSH) were administered to prepubertal lambs. Production of GnRH- induced less acidic FSH was blocked with a competitive GnRH receptor antagonist, Nal-Glu. Beginning 24 h after Nal-Glu, lambs were injected with C-FSH or N-FSH and LH. Controls included untreated, GnRH-treated, and Nal-Glu-treated groups. Blood samples were obtained at 2-h intervals. Plasma FSH levels were similar before treatment and increased over time in the C-FSH but not the N-FSH group (P < 0.001). Three of the six GnRH-treated ewes exhibited an LH surge. Peak E2 concentrations in the GnRH-treated animals were achieved 30-36 h after initiation of treatment. Peak circulating E2 levels tended to be higher in the C-FSH than in the GnRH-treated group. Only two of six N-FSH-treated ewes had a serum E2 rise. The C-FSH ewes had more estrogenic follicles than the GnRH and N-FSH groups (P < 0.05). Our findings show that C-FSH clears more slowly than N-FSH, and C-FSH is a better facilitator of follicular development and maturation than N-FSH.