Naltrexone was given to ten opiate-free volunteer subjects following the same dosage schedule used for initiating treatment of opiate-dependent persons. During the three-week initiation period, three subjects dropped from the study owing to aversive effects of the drug. The remaining seven subjects reported similar unpleasant but tolerable effects. A separate group of ten volunteer subjects was given single doses of 50 or 100 mg of naltrexone or a naltrexone placebo on three separate occasions using blind controls. These subjects also reported aversive effects. The principal symptoms reported were loss of energy, gastrointestinal disturbances and mental depression. It is possible that these aversive reactions of naltrexone have limited acceptance of the drug as a treatment for opiate-dependent persons.