Influence of lithium on mammary tumor growth in vivo. 
The possibility that lithium ions stimulate growth of mammary tumors in vivo has been suggested by their mitogenic action in vitro on normal and neoplastic mammary epithelium [8] and their clinical use as stimulators of neutrophil production in tumor-bearing patients treated with cytotoxic drugs [14,15].  Three experiments were performed to assess this possibility.  Buffalo/N female rats received a single injection of N-nitrosomethylurea (NMU) at a dose known to produce mammary carcinomas in about 50% of animals under standard conditions.  Under lithium treatment, the incidence of tumors did not increase significantly.  Sprague-Dawley female rats treated with a single dose of 7,12-dimethylbenz[alpha] anthracene (DMBA), but showing no mammary tumors after 4 months, received lithium in their drinking water for 3 additional months.  The number of late-appearing tumors was not increased by lithium treatment.  Buffalo/N females with NMU-induced tumors were castrated, and the subsequent changes in tumor volume were compared in lithium-treated and control animals.  The regression-regrowth curves were not altered by lithium treatment.  These results are in contrast to the growth stimulatory capacity of lithium on mammary epithelium observed in vitro [8] and indicate it is very unlikely that lithium ions have an undesirable growth stimulatory action on primary mammary carcinomas in vivo.