Experiments were made to investigate the effect of four anesthetic drugs that are commonly used in surgical practice on the postoperative growth of mouse tumors in syngeneic recipients. These experiments revealed that some of the anesthetics when applied for surgical excision of the local tumor, strongly accelerated postoperative progression of spontaneous lung metastases produced by the 3LL Lewis lung carcinoma and by the B16 melanoma. Some of the drugs caused the appearance of metastases in organs, such as the liver, in which spontaneous metastases are not usually produced by these tumors. A T10 sarcoma clone that does not produce detectable metastases in immune intact mice even following intravenous injection, did produce metastases when injected into animals treated with pentothal sodium.