The stability of atenolol in an oral liquid stored for 40 days under various conditions was studied. A liquid preparation of atenolol 2 mg/mL was prepared by triturating 50-mg atenolol tablets with a commercially available oral diluent. Twelve 30-mL portions in vials were prepared. Three vials were refrigerated (5 degrees C) and shaken immediately before analysis, three were stored at room temperature (25 degrees C) and shaken, three were refrigerated and not shaken, and three were stored at room temperature and not shaken. One sample from each vial was assayed in duplicate on days 0, 15, 30, and 40 by stability-indicating high-performance liquid chromatography. The concentration of atenolol remained above 90% of the original concentration in each vial under each of the four sets of conditions. Microbial growth occurred in a few cultured samples, and pH changed minimally. Atenolol 2 mg/mL in an oral liquid was stable for up to 40 days when stored at 5 or 25 degrees C and shaken or at 5 or 25 degrees C and not shaken.