Abstract Altrenogest is frequently administered to young stallions in the equine industry as an off-label use to suppress sexual/aggressive behavior. In this study, 2-yr-old Quarter Horse stallions in early-performance training were administered altrenogest (0.044 mg/kg BW daily) to determine the effects on sexual/aggressive behavior, testicular parameters, and steroid hormone profiles. Horses were randomly assigned to treatment (n = 5) and control (n = 5) groups. The treatment group was administered a daily oral dose of 0.044 mg/kg BW daily for 67 d; the control group was given a daily oral sham dose of corn oil for 67 d. No significant differences were found between treatment groups in BW or body condition scores. Altrenogest had no effect on any behavioral parameters or seminal parameters measured. Averages of total scrotal width (TSW) were lower (P