The literature indicates that morphine can inhibit the growth of both small cell and non small cell lung cancer cell lines and that nicotine can reverse this inhibition. In this report we present data showing that mammalian lung tissue contain the opiate alkaloids morphine and codeine and that these alkaloids are also to be found in normal lung cell lines. However, analysis of both small cell and non small cell lung cancer cells indicate that they do not contain these opiate alkaloids endogenously. If morphine exerts an inhibitory effect on proliferation of these cells it is interesting that the lung cancer cell lines lack the opiate alkaloids endogenously. Our studies also present data indicating that the circulating levels of morphine and codeine are elevated in smokers as compared to non smokers which we hypothesize to reflect the invocation of a compensatory mechanism.