Virgin female Sprague-Dawley rats (50 days of age) were administered a single intragastric 10 mg dose of 7,12-dimethylbenz(a) anthracene (DMBA). Three weeks later they were placed on diets containing either 20% corn oil (CO), 20% primrose oil (PO), 20% black currant seed oil (BCO), 20% borage oil (BO), 15% menhaden oil plus 5% corn oil (15% MO + 5% CO), 10% menhaden oil plus 10% corn oil (10% MO + 10% CO), 5% menhaden oil plus 15% corn oil (5% MO + 15% CO) or 10% menhaden oil plus 10% borage oil (10% MO + 10% BO). Incidences of mammary tumors at 16 weeks post-DMBA were 80% in rats fed the CO diet, 84% in rats fed PO diet, 67% in rats fed BCO diet, 88% in rats fed BO diet, 60% in rats fed 15% MO + 5% CO diet, 67% in rats fed 10% MO + 10% CO diet, 83% in rats fed 5% MO + 15% CO diet, and 92% in rats fed 10% MO + 10% BO diet. Tumor multiplicity was lowest in PO-fed rats and highest in BO-fed rats. The tumor burden per tumor-bearing rat was lowest in rats fed the 15% MO + 5% CO, and 10% MO + 10% CO, diets and highest in those fed 20% BCO diet. Although body weight at 16 weeks post DMBA was not significantly different among the dietary groups, food intake was significantly greater in rats fed a diet containing 20% BO, or 5% MO + 15% CO.(ABSTRACT TRUNCATED AT 250 WORDS)