The differential distribution of a series of antineoplastic agents in metastatic tissues compared to their respective primary tumors has been investigated in one rat and two mouse experimental tumor systems, ie, the intramuscular Lewis lung carcinoma (3LL) of C57BL/6 mice, which gives rise to spontaneous lung metastases, the intratibial Sarcoma 180 (S180) of CD1 mice, which induces macroscopic metastases to the lymph nodes, and the Walker 256 carcinosarcoma of CD rats, which also metastasizes to the lymph nodes. The results described in this paper show that the concentrations of adriamycin, daunorubicin, cyclophosphamide and its alkylating metabolites, hydroxyurea, 1-methyl-1-nitrosourea, and 6-mercaptopurine are much higher in the pulmonary metastases of 3LL and/or in the lymph node metastases of S180 than the concentrations measured in the primary tumor. In the Walker 256 tumor system the distribution of adriamycin appears to follow the same pattern observed for the mouse tumors. Only for methotrexate (in the 3LL tumor) is the difference in the concentrations at the two sites not so evident. These findings are discussed in relation to the comparatively greater sensitivity of metastases to chemotherapy.