Tamoxifen was given orally to 60 puerperal women to inhibit lactation. Twenty control puerperal women were given placebo. Fifteen women receiving tamoxifen and 15 women receiving placebo were studied before, during and after the use of a breast pump under basal conditions and after five days of treatment. Tamoxifen was effective in inhibiting lactation; no rebound phenomena were observed. Its administration was free from side effects. This drug was capable of preventing the prolactin release induced by mechanical breast stimulation. Placebo failed to inhibit lactation and had no effect on prolactin release induced by the use of a breast pump.