The in vivo measurements of 127Xe and 79Kr clearance from the whole body indicate five components, with biologic half-times ranging from 13 sec to 17.04 h. The slowest component correlated highly with percent total-body fat, the half-time varying between 7.59 and 17.04 for xenon and between 4.20 and 9.62 h for krypton. The long-term retention was localized to regions of high fat content as determined by the coincidence mode of the whole-body counter and images obtained with the scintillation camera. The estimated dose based on the retention data of this study would indicate increased doses to body fat and gonads as compared with those previously calculated. These increased doses are due to the longer retention of noble gases in fat. After 10-min inhalations of a mixture of 127Xe and air and 79Kr and air, approximately one third of the total 127Xe and one fifth of the total 79Kr were transferred to the body tissues, extrapolated back to the start of gas washout. Of this amount, approximately 13% of the 127Xe and approximately 9% of the 79Kr were associated with the slowest component.