Differentiating agents have been used experimentally and clinically as an adjuvant in the treatment of cancer, but their role in chemoprevention is limited. We used 5% dimethylsulfoxide (DMSO), 1% and 4% methylsulfonylmethane (MSM), 0.3% N-methylformamide (NMF), and retinol acetate (RA) in the chemoprevention of rat mammary breast cancer. One hundred fifty 42-day-old Sprague-Dawley rats were randomized into six groups (control, RA, DMSO, 1% MSM, NMF, and 4% MSM) and received chemopreventive agents along with standard rat chow ad libitum. Eight days later, 15 mg of 7,12-dimethylbenzanthracene was given by oral gastric intubation. The animals were examined weekly for tumor incidence and size (biplanar analysis). Animals were followed up for 240 to 300 days. Tumor incidence was not statistically affected. Time to appearance (latency period) of both tumors and cancers were prolonged by NMF, DMSO, and 4% MSM. Doubling times of all cancers produced were prolonged by DMSO and RA. No group exhibited toxic reactions or significant weight loss. Polar solvents and differentiating agents, specifically NMF, DMSO, and 4% MSM, were effective in the chemoprevention of dimethylbenzanthracene-induced mammary cancers.