Effect of chemopreventive agents on mammary tumor formation in the NMU rat model of mammary tumorigenesis. Rats were administered NMU at 8 weeks of age (time t = 0), and were randomized into six groups which were given the following chemopreventive agents continuously in their diet, starting 1 week after initiation: no additions (unfilled circle), 4-HPR (filled circle), 9cRA (unfilled square), tamoxifen (filled square), 4-HPR + tamoxifen (unfilled triangle), or 9cRA + tamoxifen (filled triangle). Rats were weighed and palpated for tumors each week. (a) Percentage of rats with palpable tumors. (b) Mean number of tumors per rat. (c) Mean weight of rat. The open arrow indicates the start of the chemopreventive intervention.