Locoweed, Astragalus lentiginosus was extracted with ether (EE) and water (WE). These and the plant residue were mixed with basal diet (BD) and fed to weanling female mice to determine the plant's estrogenic activity. Diethylstilbesterol (DES) was fed as a control. Uterine weight was taken at five to seven days. Three feeding trials were conducted. In Trial 1 one group of mice was fed a basal diet and one group the basal diet plus EE 1 gm/mouse. The uteri from mice receiving EE were heavier (P <.01) than from those receiving BD. In Trial 2 mice were fed 0.10 gmEE/mouse, 0.15 gm EE/mouse, 0.10 and 0.15 microg DES/mouse and basal diet. The uteri from the DES fed mice were heavier (P <.01) than the others. The uteri from the EE mice and the BD mice were lighter than those of the DES mice, but there was no difference between the uteri from the EE and BD. This was thought to be due to the level of EE fed. In Trial 3 three levels of EE, 1.30, 0.86, and 0.42 gm/mouse, two levels of DES, 0.075 and 0.150 microg/mouse, one level water extract, 3 gm/mouse, plant residue, 3 gm/mouse, and basal diet were fed. The uteri from EE mice were heavier (P <.01) than the DES and DES heavier (P <.01) than WE or BD fed mice. Lesions typical of locoweed poisoning were found in WE fed mice. It was concluded that locoweeds contain high levels of estrogenic compounds as well as those causing pathological lesions.