The mechanisms by which estrogens and antiestrogens modulate breast cancer growth have not been totally defined. We have examined the cell cycle kinetic effects of estrogens and antiestrogens in cultured human breast cancer cell lines. In a previous study, we showed that tamoxifen induces a transition delay in early to mid-G1 phase of the cell cycle. In the present study, we show that this cell kinetic alteration by tamoxifen is dose dependent and that other antiestrogens have identical effects. As little as 0.01 to 0.1 microM tamoxifen reduces the S and G2 + M fractions and increases the G1 fraction of MCF-7 cells growing in medium with 5% charcoal-stripped bovine serum. More than 90% of cells are in G1 72 to 96 hr after the addition of 1 microM tamoxifen, a concentration achieved in patients treated with the drug. Nafoxidine and trioxifene have identical activity. Partial reversal of tamoxifen growth inhibition is observed with a simple change to tamoxifen-free medium. Complete reversal of the tamoxifen effect is observed with the addition of 17 beta-estradiol. By 24 hr after the addition of estrogen, 60 to 70% of tamoxifen-inhibited cells have progressed through G1 and into S phase, indicating that tamoxifen-treated cells remain viable. This estrogen "rescue" effect is observed even in the absence of a change to tamoxifen-free medium. A 100-fold-lower concentration of estradiol can totally reverse the inhibitory effects of 1.0 microM tamoxifen. Stimulation of the progression of G1 cells to enter S phase is also observed when estradiol is added to cells maintained for four days in medium with stripped serum, even in the absence of tamoxifen. Similar effects are observed in the estrogen receptor-positive ZR75-1 breast cancer cells. No effects of antiestrogens or estrogens are observed in the receptor-negative MDA-231 cells, suggesting that these effects are mediated through the estrogen receptor. In summary, antiestrogens and estrogens have prominent effects on the cell cycle kinetics of endocrine-dependent human breast cancer cells. Antiestrogens cause an accumulation of cells in G1 phase. Estrogen reverses this block with a synchronous cohort of cells progressing through the cell cycle. These data have important implications for the design of rational clinical trials of combined chemoendocrine therapy.