This study was designed to determine whether intravenous infusion of recombinant human insulin-like growth factor 1 (IGF-1) stimulates tumor growth. In order to determine the potential interaction between nutrition and IGF-1 administration the study was conducted in fasting rats and during continuous feeding by total parenteral nutrition. Tumor cell cycle kinetics including labeling index, DNA synthesis time, cell cycle time in Go/G1, and G2/M in the total cell cycle, and potential doubling time were determined by flow cytometry after in vivo pulse labeling the rats bearing the Walker-256 Carcinosarcoma with 5'-bromo-2'-deoxyuridine (BrdUrd). The results show that IGF-1 treatment has no significant effects on the proliferative characteristics of the tumor model regardless of the feeding status of the animal. This study provides preliminary cell-cycle kinetics data on the short-term effect of IGF-1 on tumor growth. Failure to show a significant effect of IGF-1 on the proliferative characteristics of the tumor suggests that IGF-1 may be given to cancer patients in amounts sufficient to promote weight gain without deleterious stimulation of tumor proliferation.