Ovarian follicular development is regulated by systemic as well as autocrine and paracrine factors such as estradiol (E2). E2 is produced by growing follicles via aromatase conversion of androgens into estrogens. Both FSH and IGF-I induce aromatase activity in granulosa cells (GC) of several species. To further evaluate the effects of FSH, IGF-I and testosterone on abundance of aromatase (CYP19A1) mRNA, GC from small porcine follicles (1 to 5 mm) were cultured for 2 d in medium containing 5% fetal bovine serum and 5% porcine serum followed by 48 h in serum-free medium containing control (no addition), FSH (30 ng/mL), or FSH plus IGF-I (30 ng/mL) in the absence or presence of 500 ng/mL of testosterone in a 2 × 3 factorial design. Cellular RNA was isolated and relative CYP19A1 mRNA abundance was measured using real-time PCR with 18S rRNA as a housekeeping gene. Data were analyzed via GLM procedure of SAS. Alone, FSH increased (P < 0.05) CYP19A1 mRNA abundance 63-fold. Addition of IGF-I to FSH-treated cells increased CYP19A1 mRNA abundance 6.5-fold above that seen with FSH alone. Testosterone alone increased (P < 0.05) CYP19A1 mRNA abundance 30-fold and further amplified both FSH- and FSH plus IGF-I-induced CYP19A1 mRNA abundance by 3- to 4-fold. In a second experiment, GC were cultured as in the first experiment except that medium (containing 500 ng/mL of testosterone as E2 precursor) was collected after 1 and 2 d of treatment for E2 measurement via RIA. FSH increased (P < 0.05) E2 production by 1.3- and 1.5-fold after 1 d and 2 d of treatment, respectively. IGF-I increased (P < 0.05) FSH-induced E2 production by 4.7- and 9.2-fold after 1 d (0.19 vs. 0.94 ± 0.06 ng/10⁵ cells) and 2 d (0.21 vs. 1.94 ± 0.09 ng/10⁵ cells) of treatment, respectively. In conclusion, testosterone-amplified basal and hormone-induced aromatase gene expression indicates that androgens produced by theca cells may act as a paracrine factor to increase aromatase activity in GC in addition to serving as E2 precursors for GC. This intra-ovarian regulatory mechanism may be vital for optimal follicular growth in pigs.