Ethynylestradiol (I) [57-63-6] was coupled with diazotized m-nitroniline and the resultant yellow color determined by colorimetry at 450 nm. I was extracted from Orlest [8015-12-1], Norlestrin [8015-12-1], and Ovulen [8056-92-6] tablets with EtOH and determined by the technique with good I recovery. The relation between absorbance and concentration was linear to 20 μg/ml.