This study investigated whether the growth of transplanted mammary tumors is altered in rats by treatment with the antimalarial drugs chloroquine (CQ) and quinacrine (QN). Female inbred F344 rats were divided into three experimental groups. Animals were injected i.p. with either CQ, QN or normal saline for 5 days a week throughout the entire experimental period (25 days). After 7 days of drug treatment each rat received subcutaneously one 2-mm2 aliquot of R3230AC mammary adenocarcinoma in the mid-thoracic region. Eighteen days after implantation, all rats were sacrificed and tumors were excised, weighed and measured. The results indicate that weights and volumes of tumors as well as tumor-to-body weight ratios were significantly higher in CQ and QN-treated animals than those in saline-treated animals. The final body weights of rats treated with QN were significantly lower than those treated with saline. The prostaglandin E2 content of tumors was significantly reduced by CQ treatment. Erythrocyte glutathione reductase activity coefficient and reduced glutathione concentrations remained unaffected by both treatments. These results suggest that CQ and QN have significant stimulatory effects on the growth of mammary adenocarcinoma in rats.