Abstract: Certain peptides administered during chronic morphine treatment were found to be effective in preventing development of tolerance and physical dependence. Particularly effective are prolyl-leucyl-glycinamide, cyclo(leucylglycine), and the protected peptide N-carbobenzoxy-prolyl-D-leucine, which, injected daily into mice receiving chronic morphine treatment, prevented the development of physical dependency as measured by changes in body temperature and body weight, either during abrupt or naloxone-induced withdrawal. But injection of peptide only on the last day of morphine treatment did not alter the overt signs of withdrawal. Daily injection of peptide was also effective in blocking the development of tolerance to analgesic and hypothermic effects of subsequent challenge doses of morphine. The peptide treatment did not alter the acute effects of morphine on either analgesia or body temperature. No effects on memory were noted, as evaulated in a one-trial passive avoidance task.