1. The effect of different nutritional situations on the growth of experimental Walker 256 carcinosarcoma was investigated in rats. Fifty adult male Wistar rats were randomly assigned to three groups: group NN received standard chow throughout the experiment; group DD initially received an isocaloric protein-free diet, and some of the animals in this group (DN) were fed normal chow from the 37th to the 53rd day of the study. 2. On the 42nd day of the experiments all animals were inoculated subcutaneously in the flank with 2.5 to 3.0 x 10(5) viable Walker 256 carcinosarcoma cells and tumor growth and rat body weight were monitored daily thereafter. 3. Significantly greater tumor growth was detected in well-nourished (group NNT) as compared with malnourished animals (group DDT), but not in protein-depleted-refed (group DNT) animals, whose tumor growth was not significantly different from that of constantly malnourished (group DDT) rats. 4. Comparison of tumor weight and of the tumor weight/carcass weight (TW/CW) ratio showed no significant difference between malnourished and malnourished/refed animals, whereas well-nourished animals showed higher tumor weight and TW/CW ratios. 5. TW/CW curves for malnourished rats were parallel to those for malnourished/refed rats. TW/CW curves for constantly malnourished rats differed from those for well-nourished rats during the first observation period but there was no difference during the second week of tumor growth. 6. Although the protein-free diet inhibited tumor growth and refeeding enhanced it, carcass weight increased at the same rate, and therefore no change was observed in the TW/CW ratio.(ABSTRACT TRUNCATED AT 250 WORDS)