Female 3-month-old C3H mice were given sc injections of 5-mg pieces of mammary adenocarcinoma and fed a linoleate-containing (15% corn oil) diet in the presence or absence of eicosa-5,8,11,14-tetraynoic acid (TYA), an inhibitor of prostaglandin synthesis. After 6 weeks, the weights of tumors of mice fed the TYA-free linoleate diet were three to five times greater than those of mice fed the TYA-containing linoleate diet. Dietary TYA caused a reduction in the levels of arachidonate and an elevation in the levels of linoleate in mammary tumors and livers. Aspirin, another known inhibitor of prostaglandin synthesis, when added to the linoleate diet, did not affect the tumor size or the composition of fatty acids in the tumors and livers. Thus we concluded that a) the growth of mammary tumors was not related to prostaglandin synthesis but was related to the availability of arachidonate, and b) TYA was an effective inhibitor for the conversion of linoleate to arachidonate.