A series of sampling flights was made to determine the vertical distribution of HT and HTO in the upper troposphere and lower stratosphere up to 18-km altitude. The HT appears to be well mixed, but striking vertical gradients of HTO were found in the region from 10 to 18 km. These gradients, essentially constant to 18 km. were used to estimate a characteristic time of 2 years for turnover of water vapor in the lower stratosphere.