Formestane (4-hydroxyandrost-4-ene-3,17-dione) is an irreversible steroidal aromatase inhibitor with reported abuse in human sports. In 2011, our laboratory identified the presence of formestane in a horse urine sample from an overseas jurisdiction. This was the first reported case of formestane in a racehorse. The metabolism of formestane in humans has been reported previously; however, little is known about its metabolic fate in horses. This paper describes the in vitro and in vivo metabolic studies of formestane in horses, with the objective of identifying the target metabolite with the longest detection time for controlling formestane abuse. In vitro metabolic studies of formestane were performed using homogenized horse liver. Seven in vitro metabolites, namely 4-hydroxytestosterone (M1), 3β,4α-dihydroxy-5β-androstan-17-one (M2a), 3β,4β-dihydroxy-5β-androstan-17-one (M2b), 3β,4α-dihydroxy-5α-androstan-17-one (M2c), androst-4-ene-3α,4,17β-triol (M3a), androst-4-ene-3β,4,17β-triol (M3b), and 5β-androstane-3β,4β,17β-triol (M4) were identified. For the in vivo studies, two thoroughbred geldings were each administered with 800 mg of formestane (32 capsules of Formadex) by stomach tubing. The results revealed that the parent drug and seven metabolites were detected in post-administration urine. The six in vitro metabolites (M1, M2a, M2b, M2c, M3a, and M3b) identified earlier were all detected in post-administration urine samples. In addition, 3α,4α-dihydroxy-5α-androstan-17-one (M2d), a stereoisomer of M2a/M2b/M2c, was also identified. This study has shown that the detection of formestane administration would be best achieved by monitoring 4-hydroxytestosterone (M1) in the glucuronide-conjugated fraction. M1 could be detected for up to 34 h post-administration. In blood samples, the parent drug could be detected for up to 34 h post administration. Copyright © 2013 John Wiley & Sons, Ltd.