OBJECTIVE To assess pharmacokinetics (PK) and pharmacodynamics (PD) of subcutaneous (s.c.) administration of recombinant FSH in comparison with the intramuscular (i.m.) route. METHODS Open, group-comparative, randomized, multiple-dose study. METHODS Phase I Clinical Research Unit.Volunteer(s): Forty-six healthy female volunteers. METHODS All volunteers were treated with Lyndiol contraceptive pills for 6 weeks to suppress pituitary function. After 3 weeks of Lyndiol, volunteers were randomized to 75 IU, 150 IU, or 225 IU s.c. or 150 IU i.m. of recombinant FSH, administered once daily for 7 days. Serum samples were collected to determine immunoreactive FSH, LH, and E(2) levels. Ultrasonography was performed for measurement of follicular growth. METHODS FSH pharmacokinetic parameters, number, and size of follicles. RESULTS The s.c. doses tested showed dose-proportional pharmacokinetics. Subcutaneous and i.m. administration of 150 IU of recombinant FSH were bioequivalent. For the 75-IU group almost no follicles >/=10 mm were found. The mean (+/-SD) number of follicles >/=8 mm on the day of maximum stimulation in the 150 IU and 225 IU s. c. and 150 IU i.m. groups were 14.0 +/- 7.1, 14.3 +/- 8.2, and 6.5 +/- 4.7. CONCLUSIONS Pharmacokinetics of recombinant FSH were dose proportional within the dose range studied (75-225 IU). Subcutaneous and i.m. administration of 150 IU was bioequivalent with respect to pharmacokinetics, but after s.c. administration the number of growing follicles and estradiol response were higher.