Summary1. Urinary bladders of many species were analyzed for cholesterol and “fast-acting” (δ7) sterol. Rodents showed the highest content of (δ7) sterol, the dog an intermediate level, and livestock very little. In rodents and in the dog 60-84% of the δ7-sterol was 7-dehydrocholesterol. All bladder sterol was unesterified. 2. The sterols were found to be more concentrated in the mucosa of the bladder than in the muscle layer, the δ7-sterol showing a greater concentration gradient than cholesterol. 3. Tissue slices of dog and mouse bladder incubated with CH3C14OONa in vitro incorporated label into both the δ7-sterol and the cholesterol fraction, but the specific activity of the former was much (35-150X) higher.