Restricted to heavy water (99.8%), rats drank it freely for the first day, then they drank progressively less and died within 14 days. When given a choice between distilled water and heavy water (99.8%) rats avoided heavy water partly by virtue of some deleterious effects of heavy water and partly by virtue of a faint smell of heavy water with which the untoward effects could be associated. Rats did not taste heavy water. Whether a higher viscosity of heavy water plays any part could not be determined. Apparently the low intensity of any untoward effects of heavy water and the faintness of its smell could explain the erratic variability of the reactions of rats to heavy water that put it in a different category from other substances tested so far.