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<0.03) for treatment animals at the end of treatment period (d 67). By d 157 mean TSW for both treatment and control stallions were not different. Both were significantly higher (P<0.03) than pretrial values at d 157. Comparisons of testicular weight (d 157) between control and treatment groups were not different. Histological analysis of testicular tissue revealed no significant difference for the average number of spermatids per seminiferous tubule. Altrenogest treatment reduced serum estrogen levels (estradiol 17-β) by d 67 (P<0.02); however, estrogen levels returned to pre-treatment values after a 90-d recovery. Serum testosterone values were not affected by treatment. Further research is needed to determine dose and age effects of altrenogest in the stallion.