The radioactivity of the parenchymal cell intracellular lipid obtained from 200-day old multiparous animals was significantly less than that of both 50- and 200-day old virgin rats at all time intervals. Furthermore, the parenchymal cell dry, fat-free tissue of the multiparous animals ocntained significantly less DMBA-9-14C than this fraction obtained from young or old virgin rats. Since there was a decrease in both the uptake and binding of DMBA-9-14C by the mammary parenchymal cells of multiparous animals, it would appear that factors associated with pregnancy and/or lactation result in an altered susceptibility of the parenchymal cell to this carcinogen. Binding of DMBA-9-14C by parenchymal cells of old virgin rats was significantly less than that of younger animals at 3 and 6 h post feeding but did not differ statistically at the later time intervals. The possibility exists that neoplastic transformation may require the interaction between high levels of DMBA and the constitutents of the mammary parenchymal cells for extended periods of time. Therefore, the decreased exposure of the cellular constituents to DMBA could account for the decrease in mammary cancer incidence observed in older rats.