Previous studies from our laboratory have shown that intravenous (iv) infusion of pharmacologic doses of prostaglandin E1 (PGE1) inhibits "spontaneous" metastases in mice bearing Lewis Lung carcinoma (LLC). This study was done to determine the effect of iv PGE1 on "artificial" metastases. Male hybrid mice (20 g) underwent iv catheterization and were begun on continuous infusions of either PGE1 at 10 μg/kg/min or 0.9% NaCl (NS). After 2 days, each mouse received an iv injection of 105 viable LLC cells and was continued on the infusion regimens. Pulmonary metastatic nodules were enumerated and measured 12 days after injection. The iv infusion of PGE1 increased the metastatic rate 6-fold, the number of metastases ≥2 mm in diameter 7-fold, and the number of metastases ≥ 3 mm in diameter 14-fold. Thus, PGE1 demonstrates differential effects on tumor metastases, acting as a promotor of artificial metastases, in contrast to its antimetastatic effect on an intact primary tumor. When the promotional effect of iv PGE1 on artificial metastases is evaluated in light of our previous studies, it can be concluded that the inhibitory effect of PGE1 on spontaneous metastases from an intact tumor is secondary to the effect of PGE1 at the level of the primary tumor, and not in the circulation or at the target organ of metastases (lung). © 1990.