Crib-biting in horses is a repetitive behavior pattern which may involve the 
activation of both narcotic receptors and dopamine receptors in the CNS. 
Crib-biting frequency, determined in 7 nontreated horses under controlled 
conditions, was usually linear for many hours and ranged from 0.3 to 14.9 
bites/min. Intravenous or IM injections of narcotic antagonists decreased these 
rates to almost zero by about 20 minutes after the injection was given. The 
duration of the response to a single injection ranged from 20 minutes for 
naloxone to 4 hours or more for nalmefene and diprenorphine. Effective doses 
were 0.02 to 0.04 mg of naloxone/kg, 0.04 mg of naltrexone/kg, 0.08 mg of 
nalmefene/kg, and 0.02 to 0.03 mg of diprenorphine/kg. Crib-biting could be 
prevented completely for up to a week by continuous infusion of 5 to 10 mg of 
nalmefene/hr. Crib-biting resumed when the infusion was discontinued, and plasma 
nalmefene concentrations decreased to below 5 ng/ml. Doses of nalmefene as large 
as 0.4 mg/kg, IV, produced only minor side effects. These side effects included 
some passage of semifluid fecal material, intermittent penile relaxation, and 
mild sedation. Treated horses responded normally to external stimuli, retained 
their appetites, and performed appropriately when ridden. Sedation wore off 
during the course of prolonged infusions. Narcotic antagonists may provide a 
novel and effective treatment of stereotypic behavior disorders.