A single dose of 75 mg/kg 7,12 dimethylbenz[a]anthracene was administered to 50-day-old virgin female Sprague-Dawley rats and 100 days later, animals were randomized and provided with Teklad rodent chow mixed with a dose of 25 mg/rat/day ibuprofen for 35 days. Ibuprofen treatment reduced tumor volume (P < 0.05) and significantly inhibited gene expression of both cyclooxygenase- and cyclooxygenase-2 (P < 0.02). These results indicate that ibuprofen induced significant regression of established mammary carcinomas which was associated with inhibition of expression of isoforms of the gene responsible for prostaglandin production.