Walker 256 carcinosarcoma cells produce subpleural pulmonary metastases when given intravenously to the Sprague-Dawley rat. The number of metastases increases when the rat is given morphine subsequent to the tumor load. The increase in the number of metastases can be blocked be pretreatment with the opiate antagonist naloxone. Naloxone itself does not influence the number of metastases. Pentazocine is an opiate that is agonistic to the endorphin kappa-type opiate receptor and partially antagonistic to the mu-type receptor, where morphine acts primarily. While pentazocine alone has no influence on metastases and may decrease the number when given early, pentazocine partially blocks the metastatic inducing effect of morphine.