For two groups of male C3H mice an eastbound transmeridional flight was simulated by inducing a time shift of the L:D schedule of 8 hr. The assumed flight brought about a maxima) reduction of the daily light and dark span, respectively. A third group remained unshifted. At seven different times during the following day, subgroups of the time shifted mice as well as of the group with unchange schedule were exposed to whole body X-irradiation. Mortality and body temperature of each animal were registered for 30 days following exposure and were regarded as indicators of radiation response. Radioresistance was found to be highest during the second half of the daily light span, confirming earlier reports by other authors. Well defined effects of the time shift and a corresponding shift of the acrophase of radioresistance could be demonstrated. There was no significant difference between the two time shifted groups, but there was a consistent slight trend towards an advantage for the group whose L:D shift resulted in a maximally reduced dark span.