Estrogen has an important role in regulating tendon collagen synthesis in women and synthesis may be reduced in postmenopausal women. Genistein, an isoflavone found in plants, can bind to estrogen receptors and cause effects in the body similar to estrogen. We hypothesized that genistein's estrogen‐like properties may aid in maintaining collagen content in oophorectomized (OVX) female Sprague‐Dawley rats. Rats were separated into 8 groups according to presence/absence of ovaries, exercise or sedentary lifestyle, and 300 mg·kg−1·day−1 genistein supplementation or vehicle. After 6‐weeks of treatment, high performance liquid chromatography was used to ascertain collagen content within each group by fluorometrically quantifying the amount of hydroxyproline present in the Achilles tendon. Collagen content was not influenced by exercise, thus groups were collapsed and comparisons made between intact (n=18) and OVX (vehicle or genistein treated, 18 rats per group) animals. The collagen content of the Achilles tendon was lower (p<0.05) in OVX rats not given genistein (OVX non‐treated: 894±8 vs intact: 1185±17 μg collagen/mg tendon dry weight). Treatment with genistein prevented (p<0.05) any decline in collagen content (1198±28 μg collagen/mg tendon dryweight) in OVX rats. Our study demonstrates that a lack of estrogen leads to a rapid decline in collagen content and genistein therapy prevents this decline.