A double-blind study was conducted to determine the abuse potential of orally administered methadone, codeine, pentazocine and placebo in ambulatory heroin addicts. Eightyeight subjects were allowed to return to a clinic daily for ten consecutive days to obtain one of the medications. Frequency of return for each drug was used as a measure of its potential for abuse. Return rates for codeine and methadone were greatest. Pentazocine had the lowest return of the four drugs, indicating that it is less liable to be abused, at least at the dose levels used.