Objective: To correlate serum and intrafollicular E2 levels in women receiving GnRH agonist and recombinant human (h)-FSH for IVF. Design: Prospective study. Setting: Academic IVF center. Patient(s): Twenty-three nonhirsute ovulatory wo men. Intervention(s): In vitro fertilization cycles were classified as high-or low-normal response, according to whether the amount of recombinant hFSH admini stered was below or above, respectively, the mean recombinant hFSH given to all women combined. Serum bioactive LH and immunoreactive FSH were measured during p ituitary desensitization preceding recombinant hFSH and repeated at oocyte retri eval. The first mature follicle aspirated from either ovary had a mean diameter >15 mm. Main Outcome Measure(s): Serum and follicular fluid (FF) insulin, bioact ive LH, immunoreactive FSH, and steroid levels. Result(s): Basal serum gonadotro pin levels were comparable between female groups. Serum immunoreactive FSH level s, but not bioactive LH levels, at oocyte retrieval were greater in cycles of lo w-normal vs. high-normal response, whereas maximum serum E2 levels and total o ocyte counts were similar between both groups. Despite elevated FF immunoreactiv e FSH levels, FF E2 was decreased in low-normal response cycles. Intrafollicula r T and P also were decreased and increased, respectively, in cycles of low-nor mal response, although FF bioactive LH levels were comparable in both groups. Co nclusion(s): Intrafollicular E2 is decreased in cycles of reduced ovarian respon siveness to GnRH agonist and recombinant hFSH and is not predicted by maximal se rum E2 or number of oocytes retrieved. Objective: To correlate serum and intrafollicular E2 levels in women receiving GnRH agonist and recombinant human (h) -FSH for IVF. Design: Prospective study. Setting: Academic IVF center. Patient (s): Twenty-three nonhirsute ovulatory wo men. (s): In vitro fertilization cycles were classified as high-or low-normal response, according to the amount of recombinant hFSH admini stered was below or above, respectively, the mean recombinant hFSH given to all women combined. The first mature follicle aspirated from either ovary had a mean diameter> 15 mm. The main Outcome Measure (s): Serum and follicular fluid (FF) insulin, (s): Basal serum gonadotro pin levels were comparable between female groups. Serum immunoreactive FSH level s, but not bioactive LH levels, at oocy te retrieval were greater in cycles of lo w-normal vs. high-normal response, while maximum serum E2 levels and total o ocyte counts were similar between both groups. Despite the elevated FF immunoreactivation of FSH levels, FF E2 was decreased in low-normal response cycles. Intrafollicula r T and P also were decreased and increased, respectively, in cycles of low-nor mal response, although FF bioactive LH levels were comparable in both groups. Co nclusion (s): Intrafollicular E2 is decreased in cycles of reduced ovarian responsive to GnRH agonist and recombinant hFSH and is not predicted by maximal se rum E2 or number of oocytes retrieved.