Noncurative excision of a primary sc Lewis lung tumor performed on Day 7 or later results in an increase in the thymidine index and growth rate with minimal changes in the cell cycle parameters of the lung metastases. The stimulation of the lung nodules is accompanied by a small but consistent decrease in median lifespan. Sham surgery performed on Day 3 or later also results in a decrease in median lifespan and an increase in the thymidine index of the undisturbed primary tumor. Artifical metastases (10(6) cells iv) are inhibited by the presence of a second (sc) implant and the median lifespan of the doubly implanted mice exceeds that of mice bearing iv implants only. In mice bearing widely metastasized Lewis lung carcinoma, surgery alone may have a detrimental effect on life expectancy, but the residual tumor foci, stimulated to more rapid growth, should be appropriate targets for adjuvant chemotherapy.