Women are at greater risk than men for certain kinds of diseases and injuries, which may at least partly be caused by hormonal differences. We aimed to test the hypothesis that high in vivo exposure to estradiol has an inhibiting effect on collagen synthesis in tendon, bone and muscle. Two groups of young, healthy women similar in age, body composition and exercise-training status were included. The two groups were either habitual users of oral contraceptives exposed to a high concentration of synthetic estradiol (OC, n=11), or non-OC-users tested in the follicular phase of the menstrual cycle characterized by low concentrations of estradiol (Control, n=12). Subjects performed one hour of one-legged kicking exercise. The next day collagen fractional synthesis rates (FSR) in tendon and muscle connective tissue were measured after a flooding dose of [¹³C] proline followed by biopsies from the patellar tendon and vastus lateralis in both legs. Simultaneously, microdialysis catheters were inserted in vastus lateralis and in front of the patellar tendon for measurement of insulin-like growth factor I (IGF-I) and its binding proteins. S-PINP and urine-CTX-I were measured as markers for bone synthesis and breakdown, respectively. Tendon FSR and PINP were lower in OC compared to Control. An increase in muscle collagen FSR post-exercise was only observed in Control (P<0.05). Furthermore, the results indicate a lower bioavailability of IGF-I in OC. In conclusion, estradiol administered as OC had an inhibiting effect on collagen synthesis in tendon, bone and muscle connective tissue, which may be related to a lower bioavailability of IGF-I.