This study examined the suppressive effects on sexual and aggressive behavior and seminal parameters in young stallions as a result of daily administration of altrenogesta for a period of eight weeks. These parameters were also examined after a recovery period of eight weeks (day 111). At three times in the study (days -9, 60 and 111), measurements were recorded including weight, scrotal circumference and body condition score of the stallions. Stallions in the two treatment groups did not differ in weight, body condition score or metacarpal measurements throughout the study. However, scrotal circumferences decreased (P < .05) in treated stallions at week eight and remained lower (P < .05) after eight week post cessation of altrenogest (week 16). Values for scrotal circumference were different (P < .05) between control and treatment groups at week 16. Decreases in estimated daily sperm production (DSP) were also found in stallions within the treatment group at both eight weeks (P < .01) and 16 weeks (P < .05). This decrease in estimated DSP was most likely correlated with decreased scrotal circumference. Spermatozoal abnormalities were significantly increased within the treatment stallions, with head abnormalities (P < .009) and tail abnormalities (P < .001) increasing between the pre-trial collection period and the eight-week collection period. Stallions were scored for sexual and aggressive behaviors using a defined teasing test three times during the study (days -12, 57 and 111). Behavior was evaluated by exposing the stallion to a hand-held mare in estrus for a three- minute observation period. Flehmen frequency and duration decreased (P < .05) among altrenogest-treated stallions at the eight-week collection period. The duration of dropping of the penis tended to decrease (P < .08) at week eight in the treated stallions. Erection duration (P < .05) and frequency (P < .005) were also reduced in the treated group at eight weeks. These data suggest that the treatment of young stallions with altrenogest for eight weeks has marked effects on sexual and aggressive behaviors and sperm production. Several of these parameters did not return to pre-trial values by eight weeks after cessation of treatment. Further research is needed to investigate the capacity of young stallions to recover over time from the adverse effects of altrenogest administration as demonstrated by reduced libido, scrotal measurements and daily sperm production documented in this study.