To evaluate the roles of FSH and LH in follicular growth, GnRH-immunized anestrous heifers (n = 17) were randomly assigned (Day 0) to one of three groups (n = 5 or 6). Group 1 received i.m. injections of 1.5 mg porcine FSH (pFSH) 4 times/day for 2 days; group 2 received i.v. injections of 150 μg pLH 6 times/day for 6 days; group 3 received both pFSH and pLH as described for groups 1 and 2. After slaughter on Day 6, measurements were made of follicle number and size, and follicular fluid concentrations of progesterone (P₄), estradiol (E₂), and aromatase activity. Injection of pFSH increased (P < 0.01) the serum concentrations of FSH between 12 and 54 h. Infusion of pLH increased (P < 0.05) mean and basal concentrations of LH and LH pulse frequency. Serum E₂ concentrations were higher (P < 0.05) for heifers given pFSH pLH than those given either pFSH or pLH alone. There was no difference (P ≥ 0.24) between treatments in the number of small follicles (<5 mm). Heifers given pFSH or pFSH pLH had more (P ≤ 0.02) medium follicles (5.0–9.5 mm) than those that were given pLH alone (none present). Heifers given pFSH pLH had more (P = 0.04) large follicles (≥10 mm) than those given either pLH or pFSH alone (none present). Overall, only 1 of 35 small follicles and 2 of 96 medium follicles were E₂-active (i.e., E₂:P₄ >1.0), whereas 18 of 21 large follicles (all in the pFSH pLH treatment) were E₂-active; of these, 8 of 18 had aromatase activity. Concentrations of E₂ and E₂ activity in follicular fluid were correlated (r ≥ 0.57; P < 0.0001) with aromatase activity in heifers given pLH pFSH. In conclusion, pLH failed to stimulate follicle growth greater than 5 mm; pFSH stimulated growth of medium follicles that were E₂-inactive at slaughter and failed to increase serum E₂ concentrations; whereas pFSH pLH stimulated growth of medium follicles and E₂-active large follicles, and a 10- to 14-fold increase in serum E₂ concentrations.