The selective estrogen receptor modulator (SERM) and an agent for the prevention of osteoporosis in postmenopausal women, raloxifene (Ral), decreased high-dose methotrexate (MTX) cytotoxicity in MCF-7 breast cancer cells. When Ral is given at least 24 hours prior to MTX, the resultant interaction is antagonistic. However, when breast cancer cells are exposed to Ral 24 hours after MTX, the interaction between Ral and MTX is not antagonistic. The proliferation of cells exposed to 10 microM Ral and 10 microM MTX alone or in combination with Ral 24 hours prior to MTX was in had the following order: MTX > Ral 24 hours prior to MTX > Ral. MTX administration 24 hours prior to Ral had the following inhibitory effects on the growth of cells: MTX 24 hours prior to Ral > or = MTX > Ral 24 hours prior to MTX > Ral > control (no drug exposure). To determine if the antagonistic interaction between Ral and MTX was a function of sequence and time, cells were exposed to Ral 24 hours and 36 hours prior to MTX exposure. The percentages of control rates were 43.48 +/- 3.90% and 54.43 +/- 2.93%, respectively, from a 24 hours and 36 hours exposure of Ral prior to MTX. The growth rates after 24 h and 36 h exposures to MTX alone were 30.30 +/- 0.61% and 33.11 +/- 2.27% of control rates, respectively. These studies suggest that: (a) the interactions between Ral and MTX are sequence-dependent; (b) Ral antagonizes the effect of MTX when Ral administration precedes MTX; and (c) Ral antagonism to MTX is a function of time.