The effects of dietary fat concentration (5 vs. 20% corn oil, 0.5 vs. 20% corn oil and fat-free vs. 20% corn oil) on the growth of the Dunning R3327-H (hormone-sensitive, well-differentiated, slow-growing) and R3327-150 (hormone-insensitive, anaplastic, rapidly growing) transplantable prostate tumor sublines were studied in Copenhagen x Fisher F1 male rats. Rats fed 5 vs. 20% corn oil or 0.5 vs. 20% corn oil showed no differences in either R3327-H or R3327-150 tumor growth. Fat-free diets had no effect on the growth of R3327-150 tumors. However, the mean weight of the R3327-H tumor at necropsy, 16 wk after implantation, was reduced by 40% in rats fed a fat-free diet compared with 20% corn oil (P less than 0.05). Energy intake, weight gain and the weight of the pituitary and prostate glands in rats bearing either tumor was lower in rats fed the fat-free diet than in those fed the 20% corn oil diet. There were no differences in serum prolactin, pituitary prolactin or serum testosterone associated with dietary fat concentration in any study. In summary, no differences in transplantable prostate tumor growth were seen over a wide range of lipid intake (0.5-20%), although a fat-free diet reduced the growth of the R3327-H prostate adenocarcinoma.